How to solve my PUK code problem?

My phone is a Sony Ericsson W890i, and yesterday I was playing around with some PIN thingy when they asked for PUK code and there's no other option so I switched off my phone and when i switched on my phone again, they asked for my PUK code, i dunno what to do. I know you must contact whatever service operator and network administrator or whatever but I dont know how to contact them!!! HELP!