Here are your parts
Parts for Frigidaire DGHS2644KF2 Refrigerator - AppliancePartsPros.com

Here is the unit's tech sheet
http://manuals.frigidaire.com/p...f/Anderson/240389638.pdf

First remove the evaporator cover in the freezer so you can see the coils.
If they are heavily iced/frosted over then it is a defrost problem.
Do not let them de-ice.

If yes.
Manually force a defrost cycle, see the tech sheet.
When the fans and compressor turn off, you are in defrost

Now check the defrost heater to see if it is on.
Be careful you do not want to burn your fingers.
If the heater is on then the control board needs replacing.

If not on.
Unplug the unit.
Remove the wire for one side of the heating element (Item 14 in Section 13) from the wiring and measure it for continuity, usually around 20 ohms or so.

If the heater is OK
Remove one wire to the defrost thermostat (Item 21 in Section 13) and measure it, should be 0 ohms when frozen. Note that it opens just above freezing so must be frozen to check it. Also inspect it, if it is bulged at all replace it even if it measures OK.

If both the above are OK then odds are the defrost contacts are toast.
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.