What exactly do you mean by "top up"?
What exactly are you trying to buy?
Membership plan, add-on, cash credit?
What happens when you try to buy it?
Any error messages?
When were you last able to successfully buy stuff on 48?
Is your payment card still valid, not frozen via your online banking, and there are sufficient funds in the account if it's a debit card?
If you need to contact 48 support about this then see the links in my signature below.
