After connecting you may have to wait a short time for the TV and PC to sync together. If none, then you have to set the TV source as HDMI port(the one the TV is connected to) and then the PC must be actually streaming something. being connected is not enough for some setups. The "active stream" being on will then be displayed from PC to TV. If it worked before then repeat the same ports being used. You gave little details to go on both my generic advise covers much of the process.
When I hook my desktop up to the tv with HDMI cable I get 'no signal' on the computer. First time got signal to tv, but when tried to go back to desktop got 'no signal' on computer. This time as soon as I hooked up HDMI got no signal on computer. No sound to tv either, but I think I have solution from these forums.