@imarkdesigns - Sounds like the person who put in the replacement drive failed to connect things up properly.
Are you up to fixing your own system? This is one of the easier things one can do on this series. Follow this IFIXIT guide here: [guide|5954]. If you are using a 3rd party drive (not Apples) you'll need this special cable: [https://eshop.macsales.com/item/OWC/DIDIMACHDD11/|OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade] as the system uses an internal thermal sensor within the custom Apple drive to manage the systems cooling. It maybe they used the correct cable but it managed to unplug on its own. I have really needed to replace the SATA or power cables on this series unless someone buggered them up.