First, try turning your phone off and on again a few times. That will reset the connection, which can often help.
If that doesn't work then there may be a problem with the network equipment in your area. Put your postcode into the Network status checker here:
If that shows that everything should be OK, there may be a fault with either your phone or your SIM / Account. Can you try putting your SIM into another unlocked (to giffgaff / O2) phone? Or put a known working SIM into your phone?
If the problem stays with your phone, then it may have developed a fault, and you need to take it to a local phone repair shop to get it checked. If you originally bought it from giffgaff, you may be able to get it repaired under warranty, see here:
If the problem moved with the SIM, try dialling the shortcodes *100# and 1007#. What messages do you get back? Is it working now?
If it's still not working, try calling the service numbers 43430 (top-up line) and 443 (voicemail). What do you hear?
If you can call 443, but no other numbers, then it is likely you have been barred. The most common reason
for this is sending too many texts, and triggering the anti spam bulk text system.
If you think that this could apply to you, you will need to contact an agent, and say that you didn't realise that bulk texting was against the rules. Say that you won't do it again, and ask for the bar to be lifted, using the form
Agents can take up to 24 hours to reply, but are usually a lot quicker. You will find their reply in the Messages from agents section at the bottom of your Dashboard page, see here:
If you can't call 443 or anything else, then it may be that your SIM has become faulty. You could also try this if you didn't have another phone or SIM to try. You can get a new SIM either by buying one from a local shop or supermarket (McColls, Co-Op, Sainsbury's to name a few) of about a pound, or by ordering one from giffgaff for free (though it can take up to 5 days to arrive), see here:
When you have the new SIM, you can do a SIM swap to transfer all your account details, including your phone number and any goodybags or airtime credit, to the new SIM. See this guide:
Make sure that you are logged into your account when you start, or you might accidentally create a new account, with a new phone number!