Internal Server Error ?!?

Hi guys. Last night I got "Internal Server Error" on every page written in php. HTML pages work just fine. Our web site is on dedicated server. I contacted tech and he said they didn't touch the server and must be something I did. I REALLY don't remember I did anything wrong. Just regular php coding. And, everything was working fine while I was in my office. I found this problem at home. What could cause this problem? Any hint? Thanks! -afan