That might indicate something is wrong with the way you installed the update for the operating system. Note you posted to the iMac PPC forum. Only early in 2006 was there a PPC iMac, which looked identical to the iMac Intel from later on. Look on the under side of the foot to determine which it is:
EMC# 2104, 2105, 2110, 2114, 2118, 2111, 2133, or 2134 are all Intel, and can boot from a Mac OS X 10.6 retail installer disc and/or data recovery software with 10.6 installed. All other EMC#s are PowerPC, and must boot from a 10.5 retail installer installer disc and/or data recovery software.
If your data is not backed up, you'll want to read this tip on trying to recover data before doing anything further:
https://discussions.apple.com/docs/DOC-1689