I have the same issue, I have prepaid M, still have 3.5 GB data left (from 6.5) to use but nothing is working.

I also received a message from o2 that my credit is almost gone. 

Not really sure how to fix this.

Hello, I am having the same problem. I still have data but I can´t use it. Also I got the same message. 

If anyone knows what is happening...I will be glad.

Thanks

Hello , @Greg011 , @sendhelp , @Voodoo.curse ,

there is an issue with the prepaid system. The technician are working hard on it. 
Please stay calm. 
If the problem is solved, you can get the amounts on your cards. 
Nobody knows how long it takes. 

Best wishes

Witti

p.a. 

Please use a translator app. 
B.w. 

Hello everyone,

thank you for your message!

Unfortunately, I have to inform you that we are currently having problems with prepaid. You can find more information about this at the following link:

Englisch translation of that:

Good morning everyone,

on Thursday morning (03.03.2022) there were technical restrictions for some o2 Prepaid customers. We are very sorry for the restrictions - they have been fixed in the meantime. We have compiled the latest information for you here:

How do the o2 Prepaid restrictions manifest themselves?

The use of internet, SMS and telephony is not possible because due to an error your credit and your booked packs are shown as used up.

What happens now?

Our colleagues have been informed and are working feverishly to repair everything for you. Of course, we will also make sure that your credit and packs are displayed correctly again as soon as possible.

Update 10:25 a.m.: Telephony is already possible again.
Update 16:15: The partial outage of the prepaid service has ended.
Update 18:33: As part of the maintenance work, the prepaid services will unfortunately be unavailable for an hour. This affects
Start of the downtime: 18:00, planned end: 19:00
Update 19:20: The work continues, we will keep you informed here.

Update 04.03. 09:10: The refund of the deducted credit has already taken place for costs incurred through telephony, costs incurred through data usage are expected to be refunded later today.

Please note: If the mobile internet has problems starting up, please try restarting your device.

The incorrectly charged telephone calls and use of data services will be refunded promptly and automatically.

What can I do?

It is not up to you, your mobile phone or your tariff. The restriction is somewhat bigger and many of our customers are affected. We have all the necessary information - so it is not necessary for you to report it. Please be patient. We will keep you updated in this article as soon as there is news.

Thank you very much for your information, messages and patience so far.

We will inform you about further details!

Kind regards
Jessica