Computer freezes, won't let me log back in sometimes, files missing?

My computer freezes everything becomes unresponsive(can't move mouse pointer etc) , when I reboot , the log in screen freezes, either stays just a black screen or when i type in my pword and press enter, it freezes. I sometimes am able to get back on if I use debugging mode, sometimes when I manage to log back in, some files are missing. WHAT IS HAPPENING