What are the chances when I go in on Monday that the computer will be spoilt?
At worst the system's power might broken, however, it's fifty-fifty. it depends the maintenance work.
Because I turned it off, so will it reduce the chances? If I did leave it on, does it increase the chances of it spoiling? Or does it make no difference?
Yes, if you didn't turn off your system then your system's OS might damage and other hardware will might broken.