You should try to power-cycle your phone, most of the times it will fix the problem, if it is still giving you issues and your phone doesn't have a simcard, you can try this step:
1.Dial on your phone *22803+SEND.
2. Listen to your phone and there is going to be a machine giving you options.
3. Choose option 1 for English
4. Choose option 1 to Program the phone.
5. You will get confirmation that it was successfully programmed.
Phone will be rebooted by itself then once is back on it should be working.