That happened to mine once (the fall, & the crunchy slider).
In my case, a screw under the screen was actually loose (it started unscrewing).
Fixed it by opening up the N900, screwing it back more tightly, & closing up the N900.

This video may help.
http://www.youtube.com/watch?v=aVEHm325-z8
Note: I didn't need to remove all 6 screws in video at time: 0:47, didn't need to remove the plastic bit at 1:16. Just be really slow & careful at 1:25, coz it's a bit tricky to put it back together there (the join between the screen & keyboard). At 1:25, you should be able to see the screws under the screen & screw them back tight.
It happened a 2nd time to me. Being a little fed up, I glued them in in the next round.

kh