Cubase 5 is slowing down the tempo by it's own!!

Help me plsssss
yesterday i saved a project, everything was fine. today i opened the same project and the tempo is slower, the vocal track sounds distorted but the the whole tracks are in place.
how do i fix this problem??

So what happens if you then try to change the tempo ‘back’ to the ‘original’ tempo?

I don’t know if a Windows ‘System Restore’ will fix it, but it wouldn’t hurt.