Below is info on this unit.http://b2b.frigidaire.com/support/ResultsPage.asp?strModel=LFFH17F3QWA+&User=&Search=Search
Pull the unit away from the wall and manually turn the defrost timer cam..
At least that is where the parts diagram shows it. You may only see the end of the cam shaft as often they are mounted on a bracket.
You will need a correct size flat screwdriver. Also it only turns in one direction.
If it fires up then the defrost timer is bad.
Unplug the unit.
Short the two wires at the thermostat together.
If the unit fires up the thermostat is bad.
Note that the above is not 100% as it may be in a defrost cycle. So if it does not fire up leave the thermostat shorted and turn the defrost timer cam.