C220 Not Starting

[jayjayjay04]

HI all am new here but here is my problem, my Mercedes C220 (2001 reg) fired up yesterday but then cut out after about 5secs and then wouldnt start,AA came out and looked at it, he couldnt figure it out so towed to a garage who cant seem to find anything wrong with the car however after banging the fuel tank with a hammer the car fired up first time, what could this be?

Hope someone can help on this

Thanks

Johnathan