Boot your phone without a SIM card (I understand you can still boot it to offline mode). Enter *#06# - what shows is your IMEI code. Check if it matches the IMEI written on the sticker in the back of the phone, under the battery. If the IMEI in the phone starts with 123456....., alas, you've screwed it up. The only way is to contact B-phreaks (
http://b-phreaks.co.uk). Download Nemesis suite from their site and be ready to shell out 30 Euro or so. That's the only way so far (other than going to Nokia itself). No operator can help you.