I have a powermac g3 desktop, running 10.2.8 with classic 9.2.2. This has happened about 6 times, and it always happens while running a classic application (If I remember correctly the applications where Arthur's Kindergarten, Blues Clues ABC's, Mia's Reading Adventure, and this last time was when starting up classic). The monitor suddenly goes to sleep, and I can't wake it up again. Sometimes I can ssh to the computer and I tried killing classic and killing the Login session, but the monitor keeps on sleeping. Sometimes I can't even ssh to the computer. The only way to fix it has been to restart. There is nothing reported by crash reporter or in any log.
P.S. yes, this started after I installed the 10.2.8 upgrade.