Lets say that PSU is good enough to supply two GTX 680's with power on a typical i5 system.
Make sure your power cable is inserted correctly into the Power supply, and if not, I believe Corsair have a 7 year warranty on most of their PSU's, in which you must be covered, unless you bought your PSU in 2005.
Are you sure its the PSU? There are a couple of ways to check.
The 24 pin cable is a very important one, too many times it can become a little bit loose and sort of go out of its socket. Check if your cables are all in properly, and if it doesn't work, then you might have a PSU problem.