AntiVirus disappearing!!!

I made my CIS do a on demand scan. After a while my computer went into screensaver mode although i did not use a screensaver so the screeen went black. After I cancceled the screeensaver mode i realised that the scanning had disappeared!!! I went to scan agian but it said the antivirus was still scanning!! I pressed Alt-tab but there was nothing!! I need help fast!!!