Dual screen problem

Hi,
I have always had my iMac set up with an HD feed to my TV, but it has suddenly decided to stop working. I have an (official Apple) adaptor converting the MiniDV output to HDMI, which is then plugged straight into my TV. I never had to set anything up, it just worked automatically. My desktop essentially just extended off the right side of the monitor and if I dragged a window or simply ran the mouse off to the right it appeared on the TV. I booted up the other day and for some reason the desktop is now constrained to the normal monitor and there's no picture going to the TV. The TV and Mac are still talking to eachother to some degree because when I turn the TV to the appropriate channel the Mac screen goes blue for a few seconds (as it always has when switching onto the 'Mac' channel).

Does anyone know how to fix this... or how to achieve the same result differently? Any advice is much appreciated, it's starting doing my head in now! Many thanks in advance.

p.s. I have OSX 10.6 and a 2.4 GHz Intel Core 2 Duo processor, all standard. But, as I say, I haven't changed or upgraded anything, everything is exactly as it was when it was working.

1. Have you rebooted the Mac?

2. Open up Display preferences and:
verify it is recognizing the 2nd monitor.
has the proper resolution.
try temporarily changing to mirrored mode by putting a check in the Mirror displays on the Appearance tab.
try using the Detect displays button.

3. Reset the SMC.

4. Try a different adapter and HDMI cable.

5. Try a different HDMI port and a different TV.

After all that and nothing works, I would be purchasing a new adapter. Make sure you get the right one. That would be a mini-DVI to HDMI adapter.

Thanks for bringing some order to things. I think it has to be the minidvi to hdmi adaptor that's gone. Off to the apple store tomorrow on Monday.