Some times ago a 2.5'' hard drive fell (luckly I hadn't nothing important inside it) and, since then, the head goes back and forth; I opened it, but I don't really know what to "touch" to repair it!
So, what happened: in this case, maybe the head "threw out of alignment"? Is it possible to recover it, "lining up" the head again?

90% head broken. Yes, you don't have to 'touch' anything to repair it.

Thanks!
Ok, but more specifically "broken" means that the head doesn't work anymore? Or, as I said, it "threw out of alignment"?

Or would it be possible to recover the hard disk changing the head?

Maybe. Q. If you 'solve' problems why ask? A diagnose should be performed.

Ok
I said that the head could have "threw out of alignment" 'cause, as far as I know, the head stat at very close distance from the surface of the platter, so, maybe a hit could have moved it

When off, heads are parked on ramp or landing zone, when spinning, the clearance between head and platter is in the microns range. Heads are made like silicon chips, can't withstand extreme shock. The PLATTERS can rotate relatively if more than one or become eccentrical due to shock. This is more difficult to fix. Or even, the axis bents developing vertical instability.