I can get the laptop display onto the screen but when I click Mirror Display it doesn't work and loses the signal. I have a MacBook Pro and the TV is a Samsung. I'm connecting via an HDMI cable. Any ideas? After all the videos I have watched when I click mirror display it should work.
It seems to me that the frequency you have your laptop configured at is not supported by your TV.
When using mirror displays, it usually mirrors your main screen (resolution & frequency). Your main screen would be the one where you have the Dock and Menu bar.
Can I ask why would you want to mirror display on your TV? Is this for watching videos? If so, I would recommend you to do the following:
With VLC you can use your second screen on full screen mode without the need of mirroring.
You are definitely using an adapter for the Mini DisplayPort in your MBP so check the adapter for functioning or use a known good adapter to test. I have done what you want to do many times even with mid 2010 laptops so I'm sure your MBP can handle HDTV resolutions just fine, either 720 or 1080. Try making sure your TV is set to one of those two resolutions, and not set to 480p. If all else fails you can try a Mini DisplayPort to VGA adapter if your TV has a VGA input (usually called "PC") on HDTV's. You won't have audio from the TV but you'll still get full resolution. Without knowing your specific model of MBP you probably aren't getting audio anyway through the HDMI adapter. Tell us what you are seeing in the Display preference pane in System Preferences.