Can a crash cause magnet came loose in a motor bell? And is it possible to fix or do i need a new motor?

Yes magnets can come loose in crashes. You can try sticking that magnet back in the bell with Epoxy or CA glue

I have had several magnets dislodged in crashes. If they are not cracked or broken, they can indeed be re-seated and glued in with CA. Don't use anymore then you absolutely need to or you can throw the motor out of balance. Be sure magnet is fully seated and does not scrap the stator as the bell rotates. It wont always work, but it's worth a try before you buy new motors assuming the motor is in otherwise good condition. Good luck!

For some motors you can get replacement bells, but if you crash hard enough, there's a chance the shaft is bent anyway... Might be worth it though for more expensive motors.

I remember that Oz on rcg reported a bad late batch of rcx2206 v2 motors, where magnets would slip even without crashing, seems they used a wrong type of glue for a while. V3 are fine again, my early batch v2's also never had a problem.