You should probably check the service status in your area, which you can do by entering your postcode at http://giffgaff.com/coverage-and-service
This should give you details of any local issues, including planned and emergency engineering work, as well as indicating what sort of signal strength you should expect, and the quality of mobile internet connection. You can also check for specific mast issues and service updates there.
If you find that there are no issues reported in your area, try:
1. Checking for any GiffGaff-specific issues in the Service Updates forum athttp://community.giffgaff.com/t5/Service-Updates/bd-p/Notice_board
2. Turning your phone off, removing the SIM, waiting five minutes, then putting the SIM back in and turning your phone back on again, then wait five more minutes and try using it as normal
3. A manual roam (which can often sort out general communication issues) - there's a handy guide for this at http://is.gd/manualroam