File manager disappeared, system hang, reinstall required

Hello!

Just using ubuntu dde normally, suddenly my file manager disappeared. Not in the app list, couldn’t find it anywhere! I tried to reinstall it and it went ok. got into File manager again! Reboot, now my system has completely hung. Login prompt at desktop boot up, doesn’t recognise my keyboard/mouse/external keyboard. Cannot proceed further. Had to reinstall ubuntu dde. How did this happen??