Firstly, are you sure that the transmitter hasn't been switched off? Your previous posts suggest that you're somewhere in Ireland and I know that some people in Ireland receive UK TV stations from transmitters in either Wales or Scotland. A lot of Scottish analogue transmitters are currently being closed down:
http://www.ukfree.tv/closedown.php
If that's not relevant to you (perhaps because you're trying to receive RTE anyway), start by taking a look outside at your TV aerial. It might have been blown down, or out of position, by high winds.
Then check all of the connections between your aerial and your TV. Simply moving the TV to dust around it can sometimes cause the aerial plug to fall out of the socket. If you've got an aerial amplifier (= 'signal booster') between your aerial and your TV set, check that it's plugged in.
As long as your TV set hasn't had any channels retuned (and you're sure that you've pressed the button for a channel you can usually view) the problem you describe is almost certainly related to insufficient signal strength, which is why I've suggested checking your aerial and its connections. If in doubt, try your TV set with a neighbour's aerial.
Chris