While your phone is still switched on, take the battery out and leave it out for about 20 minutes. It doesn't matter about the sim card, that can stay
If that doesn't work, try doing a manual network select on the phone. If that STILL doesn't work, call 02 and ask for a coverage check to see if there have been any issues with the masts in your area. It may be down to coverage or congestion on the network over the last few days...
Hope that helps.
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Now: K6 GSX-R 750 with Ti Scorpion can, Tail Tidy, Shorty Pazzo Levers, Pipercross Air Filter. Lushness
Ex: SV650 Naked in Pearl Nebular Black