You must first find out what is wrong with it - they are quite simple things and the only problem areas are the inlet valve or more commonly a puncture.
If it's the valve at fault you may not be able to repair it - if a regular puncture then a repair kit may be had from an outdoor equipment shop - the kit will include instructions, patches and adhesive.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix camping equipment.