Nice one, if you did upgrade to OS 5.0.xxx, something probably went very wrong and you had better revert to OS 4.6!
Otherwise, if you are sure the SIM card is OK, proceed with the next steps.
1. Remove any suspicious software you may have recently installed
2. Disconnect USB cable, earphone and charging cradle
3. Switch off bluetooth and wifi in the connections screen
4. Make sure the battery is fully charged
5. Activate the factory theme
6. Now, reset the BB (battery pull) so that it starts up as clean as possible.
If it still does not work, return it to the shop where you bought it and have it replaced...
Posted from my CrackBerry at wapforums.crackberry.com