Your phone might have switched to offline mode accidentally if your keypad is not locked and you have the phone in your pocket/bag.
Press the power button momentarily to bring out a list of profile options, and select "General";
or you can go to Menu -> Settings -> Profiles
and choose Normal Mode;
or you can turn the phone completely off and then turn it back on; while the phone boots up you will be asked if you want to exit offline mode -> answer yes
In order to prevent the phone from switching modes, or making inadvertent calls, remember to lock the keypad when the phone is not in use:
Left selection key + functions key