2000 Jetta intermittent starting problem

My daughter has a 2000 Jetta that has a intermittent starting problem. The car will work fine for weeks then will not start, turn the key and nothing. Some times you can leave the car sit and come back later and it starts. I have also had some luck or just timing when I tap/beat the fuse box over the battery, not sure if it has anything to do with the problem. Thanks in advance for any help.