It is possible, iMacs aren't the easiest machines to upgrade their hard drive. If you installed it yourself, bring it in to the service center to make sure you didn't miss a connection. Make sure your data is backed up first while starting from safe boot (holding the shift key). If it won't safe boot, chances are there is a more serious hardware problem, although an overflowing hard drive can be another. How much capacity does your drive have, and how full is it?