Which makes of TV set is it? (I'll take a guess at Panasonic!)
What (other than an aerial) have you got feeding a signal to it? (e.g. a Sky box, a DVD recorder or whatever)? If you've only got one device (or none at all), try disabling the automatic switching between devices. (That's what 'EC' means on a Panasonic set; it stands for 'External Control'). You'll need to hunt for it within the menu settings or, if you tell us the exact make & model, we might be able to find the instructions for you.