If it is a slipped tendon, none of the things you have tried will work. You have to get the tendon back into the groove on the back of the hock and secure it there. You will probably need to fashion a splint and secure it in place.
The earlier it is addressed properly the better the chances...
Photos may help. Sounds like a slipped tendon. If so, you have to manipulate the tendon back into the groove on the back of the hock and secure it there. It will likely need a splint and taped in place. The prognosis for recovery is not good.