Hi Morgan97,
A few things to check:
1. Try out a friend's SIM card to see if you can go onto your friend's subscribed network.
2. If no, then try taking out the battery for 30 seconds, and start up again.
3. If the problem persist, try to perform a restore to factory default, which should be available as an option under the settings menu; data should remain intact, but there's no harm doing a backup.
If the problem persist after step 3, you may want to bring your mobile to Nokia Care Centre for them to take a look at it. Do remember to bring your mobile receipt (I'm assuming you purchased the mobile first hand).
Hope this helps. Let us know the outcome.
Cheers.