You'll need to post a bit more info than that... Were you able to before? What happens when you try? Any error messages? How are you trying to buy membership? On the app, via the website, by text? Can you try a different way to see if that helps? If you're trying to buy via the website can you try a different browser/computer/network connection? Are you trying to buy using your cash credit balance to pay for or using your bank card to pay for it? If the latter are you sure that your bank card is still valid?
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.