iPhone keeps freezing up and then deleting all my contacts

Shortly after I got my iPhone THIS year only 5 months ago. It has been freezing up on me and booting me out of apps. First it was my Facebook app, then my pictures, then my settings and then ultimately my messages. I end up having to cut my phone of by sliding the power button off. But when I go to turn it back on, ALL my contacts are deleted! This has happened once before and now again. I have version 5.0.1 and have heard of others with different versions doing the same thing. Can anyone tell me how to fix this problem?