My laptop has been connected to my TV (Sony Bravia KDL-52EX700) via HDMI for some time with no detection problems.
I recently got an HDMI cable long enough to permanently connect my desktop computer to the TV. Upon first plugging it in, the TV was detected in display settings just fine.
I unplugged the TV from the computer to hook another monitor up to my computer via DVI which required me to hook my main monitor up via HDMI, both worked fine.
I unplugged the second monitor from my computer, plugged my main monitor back into DVI and hooked the TV back up via HDMI. The TV wasn't detected in display settings. I tried variations of plugging the HDMI cord into different HDMI inputs on the TV, using different HDMI cords and also trying the old laptop. Neither computer would detect the TV under various conditions.
I've tried to uninstall my monitor, plug only the TV in and boot up the computer to see if it will auto-install via the wizard, but nothing ever happened. As a side note, my video card is a Radeon 7950 with current drivers (although since the laptop doesn't detect the TV either, this shouldn't be the problem).
This may be a edid problem. For computers and tvs to connect they have to recognize each other like a handshake. If one is producing the wrong signal it will not detect each other. Usually the tv is producing a viable edid signal but your pc may be not. I recommend downloading extron EDID manager. They will have more support on how to use it as it is very complex to modify EDID signals but it may also autofix the problem all together. Another solution for this problem is a HDMI detective. This will modify the signal for you but they are expensive usually around a $100.