Check your consumption on your online Customer Area or your SMS notifications. It is possible that you have used up your entire monthly mobile internet volume.
Activate your 4G or 5G option on your smartphone in the settings. It is possible that this has been deactivated by mistake. You can also try to deactivate and reactivate it.
Switch your phone off and on again to activate the automatic configuration of your device.
Test your SIM card in another mobile phone to see if there is a problem with the network or with your device.
Check the network settings of your mobile. Depending on your smartphone, go to Settings / Cellular data or Settings / Mobile networks. The connectivity options must be activated for domestic and international use.