If he is unable to straighten out it sounds as if he's been electrocuted. Be very careful but check your aquarium set up for stray water/ electricity contact.
Otherwise he may have jumped out & broken his back or come into contact with the lights inside the hood?
if there is nothing wrong with the water quality (& i'm assuming you have a mature biological filter) then a good first aid measure is a salt solution.Treatment dosages:
Prolonged immersion:
1-3 grams per litre[(1-3 ppt - 0.5%)(0.25 - 0.5 oz per gall UK)]. The lower dosage is often recommended for koi ponds as a prophylactic treatment on a permanent basis. But can be very useful for swim bladder & constipation.
Bath treatment for freshwater ectoparasites and bacterial gill disease.
10-30 grams per litre[(10 - 30 ppt - 1-3%)(1 - 4.7 oz per gall UK)] for up to 30 minutes. The higher dose may only be tolerated for a few minutes. My own preferred dosage, unless there is a reason to go higher, would be 20 grams per litre (3oz per gall UK) for 20 minutes.
�Do�s and �don�t�s...
Make sure it is fully dissolved to prevent �salt burns�
Aerate water in short-term baths
Remove fish if they �go over� as weaker ones may not stand the full treatment time
Use aquarium salt. Do not use salt containing anti-caking agents such as sodium ferrocyanide (yellow prussiate of soda � this can release hydrogen cyanide when exposed to sunlight!)