Depending on your system. It does sound like it's just a fuse. If it's quite old: you would have a on/off switch box somewhere near to the timer. This is a small white box with a red on/off switch. If you have one of these remove the cover and replace the fuse. This will cost you a few pence.
Another posiblility is that the pilot light has gone out, if you can see a flame then it's most likely to be just the fuse. If there is no flame, it needs relighting.