If the Apple Store you visited has a Genius bar, call ahead to set an appointment
for their genius or specialist to test or diagnose the computer, in case of failure.
This diagnostic testing is a free service in some locations outside the US/Canada
and does not require an AppleCare plan or warranty to be in effect. Other third-
party or authorized Apple repair service centers may offer testing at a fee rate,
so a Genius bar appointment is a fair value in troubleshooting an issue.
Correctly identifying what build model year and system your computer configuration
is, helps us volunteers try & find more information to answer questions. If your iMac
was made before 2005, it probably is a PowerPC hardware model; if made after,
it likely is an Intel hardware model. There is a difference in troubleshooting as the
system and hardware both changed in the life of the iMac name, several times.
Good luck! 🙂