There could be many reasons for this but typically it could be due to a mast being down or under maintenance in the area . This can be checked on the coverage checker depending on your network under network issues.
https://www.vodafone.co.uk/network/status-checker
https://status.o2.co.uk/
https://ee.co.uk/help/service-status
Turning your phone's connection off and then back on is the quickest and easiest way to try and fix your signal issue. If you're moving around from one location to another, toggling Airplane mode restarts the Wi-Fi, Bluetooth and cellular network modems, which forces them to find the best signal in the area.
If there are no known issues in that area, then there are other steps that can be taken to assist. Manual roams are the most effective way to get yourself back on the network, the way to do this is below.
For an iPhone it's
Press Settings.
Press Mobile Data.
Press Network Selection.
Press the indicator next to "Automatic" to turn automatic network selection on or off.
If you turn off the function, press the required network.
For a Samsung it's
From the home screen swipe down to access settings.
Tap the settings icon.
Tap Connections.
Tap Mobile networks.
Tap Network operators.
Tap Select manually
If this isn’t successful, then please get in touch with us either by calling 03332025000 or emailing us on [email protected]. We will need to send you out a new sim card but in the meantime you will need to remove the sim card and pop this in a different handset so we can ascertain if it is a handset or a sim card issue. If it is a handset issue, you could potentially need a proof of purchase so you can take this in to Samsung/Apple where a technician can full diagnostics on it. If it is the sim card, the new one should do the trick. If however the new sim card doesn’t work, then we will need to fill in a fault form so the network can investigate the issues you are having. You will need to provide lots of examples of your issues, screenshots, post codes, call logs all within 24 hours etc. If they can find no issue, the last step is a disconnect reconnect to see if this resolves the issue. If that still doesn't resolve your issue, there is the potential for us to switch your network however the network will never allow you to switch without everyone of these steps being processed first.