first, eliminate silly stuff, so try a different cable and a different guitar to make sure that the amp is really the problem.
next in order of suspicion would be the preamp tubes.
check page 5 of this document:
http://www.mesaboogie.com/manuals/St...022%20Plus.pdf
does either the lead or rhythm channels work, or neither? those are the sorts of diagnostics that should help you pinpoint which tube to change. also, you could just as easily get a spare 12ax7 and swap till the problem is fixed.