You should probably check the service status in your area, which you can do from http://status.o2.co.uk/ by entering your postcode and clicking the 'Check' button. This should give you 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.
If you find that there are no issues reported in your area, try:
1. Turning your phone off, then wait five minutes, then turn your phone back on again, then wait five more minutes and try using it as normal (yeah, I know "Have you tried turning it off and on again?" but you'd be surprised how often this works)
2. A manual roam (which can often sort out general communication issues) - there's a handy guide for this at http://is.gd/manualroam