My Sierra has been sitting for a good 2-3 months outside. The other day i tried starting but the starter does not turn over nor do i hear any clicks from it. I have measured the voltage from the battery and ignition and both are good. It has started perfectly the past 5 years.

 

Bridge it (carefully) and see if it spins up that way. Or do it the safe way and remove it from the car and test.
If its fine in the above, likely ignition switch.

^^I'm assuming very basics such as wiring/fuses/relays have all been checked (2-3 months is plenty of time for cold mice or rats to chew through some wires or water to creep into places it shouldnt)

 

First thing would be to verify if you have +12V at the solenoid (requires an assistant to operate starter), if yes then the solenoid/starter is the issue.

Martin