Roman, try, - menu, settings, phone, phone management, security, phone and sim card. The lock code preset is 12345. You may need your personal unblocking key (PUK) code and the PUK 2 code. These codes should be supplied with the SIM card. Unfortunately if you have had 3 unsuccessful attempts at PIN code entries, the code is blocked and you will need the PUK code to unblock it. If this is the case you will need to contact the network service provider with you SIM card number and get them to do it.
Alternatively you could try, menu, settings, connectivity, network, operator selection, manual, select network.
Hope this helps, good luck.