Ok, so if we're dealing with a Nokia (and I don't think it matters what model, as long as we're talking recent handsets) - then the first thing to work out is what message you're seeing.
if you see the words: Simcard blocked - Enter PUK
Then you may be ok. You'll still need to contact your service provider and give them your IMEI number (plus confirm some security details against the records the network have for that number).
They'll then give you the PUK (Pesronal Unblocking Key).
If you're seeing a message that looks like this:
Simcard rejectedThen the bad news is you'll need to purchase a brand new SIM card.