Switch it off, remove the sim and check that it's clean, now push it back in as far as it will go (use the stylus to help you).

Now switch back on.

If it asks you if you want to continue offline say no. If it doesn't ask and is still offline try selecting the "general" profile and see if it gives you any error messages.

If it still fails to work try another sim card to find out if it's the phone or your sim that is having a problem with that phone.