I have an imac and the usb ports are not working, how can I get this fixed?

if it's pretty new then under warranty the place you bought it

if it's not under warranty then at an authorised apple repair place in the country you live in

Make an appointment at an Apple Store to have your device examined by a technician. Or contact Apple Support.

Here's something you can try right away:

OS X Yosemite: Reset your computer’s PRAM

OS X Mavericks: Reset your computer’s PRAM

Intel-based Macs: Resetting the System Management Controller (SMC) - Apple Support