Remove the Prism card and boot the computer, If the Computer is ok it will display all the POST info . Reseat the Prism card and reboot if you get nothing on the screen then there is an issue with your Prism card.
While running the computer without the Prism card on the original chassis you will get a double image, nothing to worry about.

Considering all was ok before you played around, it is most likely a connection problem. Also keep in mind some of these thermal compounds ( not sure about the one you used) are conductive so you should check that nothing has been shorted by this stuff.