It could be a signal problem. Try the sim card padding trick.
- Turn the phone off
- Take a paperclip and bend it 90° outward
- Stick the free end of the paperclip in the tiny hole on the top of the phone and push straight down rather firmly until the tray pops out
- Clean off the sim card contacts with some sort of lint-free cloth
- Hold the phone upside down and blow into the slot to get any dust and debris out
- Cut up a post-it note or something into pieces small enough to fit in the tray, and place 1 or 2 pieces into it, then place the sim card on top, making sure it's lined up correctly (it may not lock into the tray after your post-it shim(s) are there)
- Pressing down on the sim card to ensure it stays in place, slide the tray back into the phone
- Main screen turn on
- We get signal?