connecting a Macbook pro to an hdmi-connected external monitor

I mis-posted this in the Mac Book Pro discussion and probably should have posted it here. We have been connecting a Macbook pro to an external monitor with an hdmi cable. We disconnected the macbook pro and connected the monitor to a tower to look for a file. When we connected the macbook pro back up, it no longer offers any options for mirroring. Any thoughts would be much appreciated.

Thank you.

Keri