It's probably too simplistic to merely say "Bryce Harper struggled for months, sat for a week and has been fantastic since." It's probably a bit of cherry-picking to also note that a struggling Justin Uptonsat for three days and has been crushing since, or that a struggling Andrew McCutchensat for three days and has been outstanding since, or that the Cubs have currently given a struggling Jason Heyward a long weekend off and are hoping for similar returns. We don't know what we don't know.
Here's what we do know: In eight games since returning from what the Nationals termed a "stiff neck," Harper -- who is set to square off against the Orioles tonight in a game featured on MLB Plus and MLB Network -- has looked a whole lot more like the hitter who terrorized pitchers in 2015 and early on in '16, and a whole lot less like the player who'd mysteriously lost nearly all of his power in May, June and July.
Consider the three parts of Harper's season, shall we?
April 1-April 30: .442 wOBA (96 PA)
May 1-Aug. 6: .315 wOBA (353 PA)
Aug. 14-Aug. 21: .518 wOBA (38 PA)
(What's wOBA? It's Weighted On Base Average, a stat that is roughly similar to batting average except that walks exist and extra base hits are weighted to be more valuable than singles. The Major League average wOBA this year is .319; Harper's mark last season was .461, meaning his April this year was nearly as good as his Most Valuable Player 2015 season.)
Or take a look in a much more visual way that shows Harper's monthly exit velocity, with August split between "pre-break" and "post-break." It's easy to see how great April was and how quickly that changed before recently.

Now, the obvious question is "what happened," and it's more than fair to point out that a mere 38 plate appearances aren't really enough to say with certainty that "he's back," because anything can happen in 38 plate appearances -- especially when three of those games came at Coors Field, and the remaining five have come against the rebuilding Braves, who have one of baseball's least effective pitching staffs. It's completely fair to point that out, because that's definitely part of the story.
Still, this isn't Sandy Leon or Ryan Schimpf coming out of absolutely nowhere to rake for a few weeks. This is Bryce Harper, a man who without hyperbole just had one of the greatest seasons in baseball history last year and was well on his way to backing it up this April. It's pretty close to unprecedented for a player to be that good, that young, and fall apart so quickly without a clear reason.
For most of the summer, baseball looked everywhere to find that reason. As we posited here in June, Harper seemed less aggressive on strikes. Others pointed out that he was hitting fewer balls in the air, or that he wasn't doing enough damage on hittable pitches, or wondered if Chicago's refusal to throw him strikes in an early-May series had confounded him. All seemed plausible. None seemed to be a reason, and when a 23-year-old defending MVP Award winner loses his power as the rest of baseball is hitting for more power, you need a cause.
In the midst of all this, a report surfaced in early August that Harper had been playing through a right shoulder injury for the previous two months, having sustained the injury on a headfirst slide. (In 2014, he missed months after sustaining a thumb injury on a headfirst slide.) The Nationals were quick to refute that, saying that the only issue was with a sore neck that popped up in early August, and we can't say with certainty that's not accurate.
Whether it was the shoulder or something else -- do remember that Harper missed several days after being hit in the knee on May 30, and you can easily understand why the team would prefer opponents not know for certain -- the injury angle makes by far the most sense, because we've seen what happens when hitters injure themselves, particularly with shoulders.

Remember how badly Yasmani Grandal collapsed down the stretch last year when it was obvious he was trying to play through a shoulder injury (he'd get surgery after the season). Remember what happened to Hanley Ramirez' debut season in Boston after he slammed his shoulder into Fenway Park's wall in early May. And while it was a foot rather than shoulder, remember how quickly Harper's teammate Ryan Zimmerman turned his year around after he finally submitted to rest rather than playing through pain. While often admirable, playing through pain doesn't always bring success.
"Now wait," you're likely saying, "even if that report of an early June injury was true, Harper began to struggle in early May," and that's a fair point. What explains that? Allow us to present a theory, using this rolling graph of Harper's swing percentage as a guide. The first peak and valley nicely correspond to April and May:

In April, Harper was aggressive, and he destroyed baseballs. That frightened teams off from throwing him strikes, as the Cubs showed and as a similar zone percentage graph indicates, and Harper, with little to swing at, stopped swinging and failed to capitalize on the rare strikes he did see. (This article from May 24 indicates clear frustration.)
As June arrived, Harper began to see strikes at a normal rate again, and he began to swing at them at a normal rate again, but, limited by something physical in early June -- the shoulder, the knee, or something else -- he simply wasn't able to do his usual damage over the summer, as other articles have proven. The sore neck perhaps exacerbated his issues, but then the break (you can see his aggressiveness returning just after game 100, which is where the week off landed) may have helped him recover.
It's a tidy narrative, of course, and a speculative one at that. Was Harper hurt before? Is he healthy now, after the time off? Without Harper's word, we can't say for sure; this just all sounds better than "he was bad for months for no reason." But what's not in question is that the Harper we've seen since his return has looked a whole lot more like the "old Harper," because the exit velo is up and so is the production. Whatever he's fixed, we may not know. If it sticks, we'll have a lot more highlights to look forward to.