Help i have a sick neon tetra?

It's ammonia burn. High contents of ammonia in the water cause the gills to bleed and "shrivel up". you can try doing a 50% water change for 3 days but if its as bad as you are saying I wouldn't hope for any recovery. Ammonia comes from rotting food, under filtered waste, over stocked tanks and more. If you have a 10 gallon tank don't put more than 5 tiny fish in it and that is being very generous. Good luck!