I just inherited the same treadmill last night.  I have come online to try and find a key & a manual.  I am using a piece a cardboard to get it to come on.  It also did the same thing you described.  All the lights came on and I was able to incline but the belt would not move.  I knew that it had not been used for several years and figured it had just froze up.  I kept turning it on and off while moving the belt manually.  After a few hours it started on its own.  I have now used it twice.  Keep trying!!!
                 Cindi
                
 January 2007