Did you try rebooting?
If you do a manual scan for networks can you see and connect to 48?
Did anything change on your phone - e.g. software update, settings changes etc.?
Do you get any error messages?
Where are you located?
Do you know any other 48 (or even Tesco or Lycamobile) users in the same area that are or are not having problems?
If necessary contact 48 support about this - see the link in my signature below.
Hope this helps.