What's wrong with all my USB ports?

none of the ports are working.the 2 front ones work but only whenever they want ...which is seldom!the back ones only worked for a short period of time but i never had problems with the other 2 in front.thats why i would always use these but lately their not working at all.i've tried connecting different devices like my camera,flash drive,printer etc.nothing has worked.tried restarting with device attached & enabling the ports through the control panel again.no luck.whats going on?please help.