Iphone 5 blocked? Keeps saying no serivce on 3 network, bought on gumtree?

I recently bought iphone 5 off gumtree with 3 network its work fine for about 2 weeks and all of a sudden it came on no service, I've tried everything, airplane mode, resetting connections and I can't seem to find the seller anywhere I'm worried I wasted my money.

Looks like you got a dud phone, only way get it looked at.