Why doesn't my GFI receptacle reset?
I would start troubleshooting by disconnecting the Load side of the GFCI, this will tell you if the problem is upstream or downstream from the receptacle.


Turn off the breaker supplying the GFCI. 
Disconnect the wires on the
Load side of the GFCI. 
Turn the breaker back on. 
Press the reset
button on the GFCI.


If the GFCI resets with nothing connected to the load terminals, you'll want to look for a short/problem in the lines and devices supplied by the GFCI. If the GFCI does not reset with nothing connected to the load terminals, you'll have to trace the line back to the breaker looking for shorts/problems along the way.