My memory is that SeaFoam claims to de-gell diesel.
Might also try Stanadyne or Sta-Bil. Others here report good results with Power Service. In any case read label for use in your situation.
Turn off fuel flow, remove sediment bowl and filter (watch for tiny spring under filter pushing it 'up' to seat), install new filter, fill sediment bowl with admixture of your choice, try to get a start.
May have to repeat as new dry filter soaks up the admixture material.
May need to crack air bleed valve and/or use decompression knob (pull knob, spin with starter, push knob in when turning fast---whole process 5-secs) repeat as needed.
Now that you know diesel can and will gell go back with some admixture your choice in every tank especially in winter. I do and never had a problem down to rare -10F.
I have a neighbor with an old Ford that uses a hair dryer to warm diesel in sediment bowl and claims it melts the wax [crystals]. Can't much recommend this but is kind of interesting.
Be aware that once melted (unless dissolved in a solvent) the wax crystals typically reform at a slightly higher temp each time they form. The wax becomes more stable as it is 'refined' or stablized by freeze-thaw cycling. Long-known problem in any hydrocarbon that contains paraffine or wax that can crystalize.
Oilfield truckers from down here go up north that I see dumping gallons of degelling material into their fuel tanks at each fill because we don't normally have heated fuel tanks in this area.
