Computer hooked up to TV but no signal

[Veziax]

I plugged my computer via HDMI to my TV but I can't get any signal. I also tried plugging it via DVI with an adapter but it still doesn't work. The TV is a 42" 720P. I then turned off the computer and unplugged the TV and plugged in a monitor and it works. I don't know what to do! Please help!

 

[Veziax]

I can also get signal when I plug in the monitor first, boot the pc up and then plug the TV but it's not very user friendly, I must say.

 

[Michael Dingus]

It may have something to do with hdcp can you post the model number and brand of your tv?