The tail seizure and the limp back legs are both signs of sever MBD (metabolic bon disease). How often have you been dusting the gecko’s food? Does the leopard gecko dust have Vitamin D3? If there is no D3 and you don't have a UV light fixture calcium is unable to be absorbed into the bones. If there is D3 and you dust often enough, how long have you had the dust? If you have had it for a long period of time (I'd say at least 2 years) it could be expired, however rather unlikely that it would lose all of its effect. How long have you had the gecko? If you haven't been the owner for very long maybe the gecko had MBD before you got it only beginning to show signs now.
In any case, sever MBD or not, you should make a trip to a good herp vet. MBD can be very tricky to treat. Unfortunately the sudden loss of tail fat may indicate the MBD has caused the gecko to stop eating and/or drinking.
The most important thing you can do for your gecko before you get to a vet is to keep it hydrated. If it's not drinking water put water on its lips forcing the gecko to lick it off.
www.drgecko.com is an amazing website for leopard gecko information, check it out before you go to vet. Not all vets know what they are talking about, this web site will help you determine if the vet is qualified or not. I know first hand how much damage an unqualified vet can cause.
Good luck with whatever the problem is.
Lisa
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0.1 Hypo Leopard Gecko
0.1 Albino Leopard Gecko
0.1 Blizzard Leopard Gecko
1.0 Albino Pacman Frog
1.0 Lovebird (Tony)
1.0 Elkhound Cross (Otto)
0.1 Malamute Cross (Paris)
6.0 Bettas