Try starting while holding down the SHIFT key. Hold that key until you get a workable desktop or see a message that says yo uare in "SAFE Mode" or Safe Boot"
I have an iMac,
You have us at a disadvantage. iMacs have been in continuous production since 1998 in four broad families. Proper troubleshooting via these forums is dependent on our know what you have. You don't want us guessing which you have and giving you incorrect information as a result!
If you don't know your sub-model and the computer won't start, give us a rough idea of when you bought it (if bought new) and, if you remember the version of the Mac OS it's running, that can help too.
You can also review these Apple articles:
to give us an idea of what you have. NOTE: The iMac G5 and the first Intel iMacs have the same white case with an integral monitor. The serial number and or an model numbers on the computers "foot" are very important.