As mentioned by Jaygeetee, most TVs have user menu options to turn TVs off after a period of inactivity (if you don't use the TV's remote, then it may shut off after say 3 hours, or some other predetermined time as mentioned above). There may or may not be a warning before this happens.
Many TVs have another option wherein the TV will turn off after a preset time (something like 10 minutes) if it doesn't get a signal. There is often a warning before this happens. I have this option "on" on my TV and the previous one "off" since I also use my TV as a monitor and my remote for the TV is not kept nearby - I don't even want a "warning" disrupting my viewing after 3 hours.
Here's a post useful for those new to the forum - FAQs, Search Tips, Optimization, etc:
http://www.digitalhome.ca/forum/showthread.php?t=57741