The answer depends on the age and make of the treadmill (you didn't specify) and what actually happens when you turn it on.
If the treadmill is otherwise working fine but just doesn't do anything when you press the incline up key, then it could be a fuse on the motor control board (depending on the age of the machine it may not use fuses so don't worry if you can't see any). If the treadmill hasn't been serviced regularly then the elevation motor may be clogged with dirt, which in extreme cases will cause it to burn out - carpet fibres and dust are the common problem here.
Newer (more expensive) treadmills tend to have more sophisticated control boards that will show up an error code on the console which will tell you exactly what the problem is, but as you don't mention any error codes i'm assuming this isn't happening.
Hope this has helped. Maybe post some details and i could offer further help.
Karl - Fitness Options Ltd, August 2007
