If you mean that you're on one of the legacy monthly membership plans with a limited mobile data allowance and you've used that up before the end of the membership month then, if you want to continue using mobile data, you need to buy a mobile data add-on:
If this is happening habitually then you might want to consider upgrading to the €12.99 plan which comes with unlimited 4G/5G mobile data allowance.
If you need to contact 48 support about this then see the links in my signature below.
