Injured Bat
If the bat is obviously injured (e.g. by a cat), it will require immediate attention from a bat carer and/or a vet.
Even if the bat does not appear to be injured, if you haven't seen the bat flying, you should not attempt to release it. It may have become starved and/or dehydrated and will benefit from short-term feeding and rehydration by a bat carer, with a view to re-release it at the location where it was found.
Call the Bat Helpline 0845 1300 228 to find out if there is a local bat rehabilitator in your area. If not, take the bat to the vet.