Has the shop told you what was wrong with it?
if it's a software fault chances are they are just reflashing the firmware. You can do this yourself using Nokia Software updater that comes as part of Nokia PC suite.
Many phone shops aren't that good at dealing with phone faults despite what they might tell you.
you can send the phone direct to Nokia and get it repaired
free using this form. In my experience they are always repaired to a very high standard (and usually fully refurbished which is a bonus) sending them here.
Normally get them back within 2-3 days after sending as well