Sigma 50 500mm OS broken ???
Exactly this happened to my 150-500OS, except the aperture control worked still in my case (but both AF and OS were gone). You can find this issue mentioned quite many times on the web for both 50-500 and 150-500.
My lens was still under warranty (in fact, this happened just few months after I bought it, which seems to be also usual behavior), so I sent it to Sigma service center and received it repaired in 3-4 weeks. Based on the servicing protocol provided, the electronics failed, namely it is described as "main PCB unit".
So sent it for servicing, there is no way to do something by yourself. In case your lens is out of warranty (hope it is not!), the price stated on servicing protocol was about 100 euros, so no that bad.