Hi @Nicolas_77064 welcome to 3Community.
I'd love to look at this for you, but I'll need a little more info. Please let me know if your mobile data was on or off when your plan expired, if you didn't manually switch it off your phone would have continued to connect to data. Using data without an active plan may be why the credit is gone.
You can check if this is what happened through your My3 account just click - My3 Account - select Allowances and View Detailed Usage.
Please let me know if you don't see any charges here or if you run into any difficulties.