What IS going on with my account

I was paying regularly and showing regular monthly statements until recently. Then on 13th March all my previous payments got credited back to me, so that I was shown as about £1000 in credit. Then on 18th March I was debited with a single debit for all my energy use over two years. And now on 6th April I have been credited with the same amount back.

And all the daily usage available since my meter became smart (last September) has vanished.

And, though this isn’t so important, it seems impossible to enter my meter reading for March 31. There is a “submit reading” button, but it just akes me back to the overview of options.