In the first instance please restart the device - where possible remove the battery for a couple of minutes and then replace before restarting.
On iPhone devices, go into the Settings and switch on Airplane Mode for a few seconds and then switch Airplane Mode off again. If this doesn't work then go into Settings, General, scroll down to Reset and select Reset Network Settings only.
If neither of these works then try the SIM card in another handset where possible - if you then have signal it is most likely to be the handset that is at fault. Please contact us to discuss options.