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If you have checked the online account and Application and have data allowance remaining, check your phone data application, this may be set too low.
It may be that the phone has temporarily lost it's connection, try going into your connection settings and complete a manual network scan, when completed select any other network, when it rejects the request, reselect Vodafone, this will update the SIM on the network. There is further information from device guides here: Device Guides
You don't mention if you are PAYG or pay monthly, if you are using PAYG total rollover, you can only roll over allowances once.