Which model iMac do you have? Do you hear any error tones (eg repeating beep, pause, beep, pause)?
If the system was working before you installed the new memory I would check the following:
Disconnect power cord (this will reset the SMC and ensure you remove/install RAM with power disconnected).
Your iMac should have room for 4 SO-DIMM's assuming you have one of the iMac's produced during the past few years. Which slots did you install the memory into? The RAM should be installed side by side as shown in the following article:
You also need to make sure the RAM is installed in the correct banks (front bank closest to the front of the iMac, or the bank towards the rear). If in doubt which to use, you can just move them from front to back, or back to front. Be sure the power cord is not connected.
It may be helpful to download the manual for your system and review the memory upgrade instructions to determine which slots to install the memory into.