it means there's no signal for your phone. sometimes phone lines get jammed.
This has happened to me before. try to do the following..
1)turn off your fone
2) remove your sim card
3) put your sim card back in after around 30 seconds
4) Turn your phone back on and see if you have coverage!
sometimes no network coverage shows up when your in a closed environment, try walking out of your building
Hope this helps
!