How can I fix my ford fiesta si?

I have a ford fiesta si 1995 M reg 1.6 and two days ago I got into the car first thing in the morning and swtiched the engine on and everything was fine. I stopped off on my way to work, when I got back into the car to switch the engine on it wouldnt start, nothing happened at all, it didnt even sound like it wanted to come on (the lights came on, on the dash board though) I managed to jump start the car but since then it wont turn on at all unless jump started. The battery has been checked and is fully charged, all the fuse's have been taken out and put back in and I cant find anything else that can be wrong. The car only had a service about 2 months ago and I have been told by the previous owner that they have never had any problems like this before. I was told that it could be the key as they can sometimes ware out but I have tried the spare key and it still doesn't work. Can anyone help me fix this problem? Thanks