You've got way too many fish for a 5 gallon tank. They probably died of ammonia poisoning. Any deal you can work out with the pet shop would be very beneficial to the fish. If you can trade them all in for credit right now, that would be ideal. If you can do that, please cycle the tank first if it's not already cycled. If you don't know what cycling is, please click on the link in my signature, and you will get some good information you will need to know for a healthy tank. Once your tank is cycled, you will probably only be able to put a betta in there with maybe 3 pygmy cories or some ghost shrimp. A 5 gallon is very limited on what fish can do well in that size.