First verify the SIM is properly seated, try using another SIM to verify.
Try a reset (search the forum for precautions) using Settings>about>reset my phone
If all else fails go to a Nokia Care point
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2013-01-20 14:20 - edited 2013-01-20 14:47
You can also take the phone back to the shop which sold it to you , or a branch of your network supplier if you're on a contract; both of which are supposed to have provided a phone that works.Nokia Care is an additional support, it doesn't supersede your statutory consumer rights and there may well be no Nokia Care centre anywhere near you.