Have you tried the hot keys to move the display to the other screen? You should have some Function key combination to move the main display to the secondary screen, which would be your TV. Usually it's FN + F5 or F7, but varies by laptop.
Aha, so you got sound, means your cable is sending stuff to the TV. Check the native resolution of the TV, and set the secondary display on the laptop to that. It could be that your laptop is trying to send a video signal to the TV that it can't read.
Once you connect via hdmi your television is probably being configured as a display. With windows you can go into properties and select the one you want to use. Follow these steps... Right click your desktop and goto display properties. At the top there should be multiple displays choose the one representing your hdmi.