You need cash credit to make international calls and send international texts but I guess you know that already? Presumably you had cash credit on your account when successfully making calls to the UK? What happens when you try to text? Any error messages? Did it fail when texting any UK number or just specific ones? Were you ever able to successfully text a number that no longer works? If so then you may need to contact 48 support. See the link in my signature below.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.