Cat back leg can’t put weight on

Hi my cat went missing for 3 days and he has come back and can’t put his back leg down properly and can’t walk on it any suggestions

Hello and thankyou for this excellent question.  You are describing lameness;  in some rare situations, lameness occurs because a motor ( movement) nerve has been damaged and an animal is literally unable to move a limb in a certain direction.  However, most cases of lameness ( as you will know yourself if you have ever had a stone in your shoe) are caused by pain that is felt when weight is bourne by the leg in a particular way; an animal steps differently in order to avoid it.  Unfortunately this is all I know about your cat;  they could have sustained a bite wound, which can be invisible under cat hair at first glance and come with an abscess with whole-body infection.  Or there could be a different wound or bruising and / or a fracture - major or minor - or a torn tendon or ligament or foreign body, or even damage to the toe, such as an ingrowing nail or tumour.  The onset you have described makes acute trauma sound most likely.   Three days is a long time to be in pain and a trip to the vet for investigation, pain relief and possible examination sounds to be on order.   Furthermore because we are not sure how the lameness originated, it is a good idea for the vet to check your cat for other, less obvious problems that could be going on concurrently.  Please do talk to them or their emergency team for triage if you haven't already.  Will you let us know how you get on?