Ford Ka 2000 starting problems?

It has happened for the second time.. As I was going back home from school the car started just fine, however, I stopped at a shop close by for some groceries and then the car did not start .. I have been told that the problem is not the battery. All lights in dashboard light up just fine and I managed to start the car on 2nd gear .. After I got home switched off the car, I tried to start it up again and was running just fine..

Any solutions? Does this sound costly ? thanks