I've gone into debt after topping up – even though I haven't been using gas

Did you have enough credit on your meter to cover your daily standing charges ?

You pay a daily standing charge even when you're not using gas. This comes off the credit on your meter.

If you didn't have any credit on your meter, the missed standing charges will go onto your meter as debt when you next top up.

Are you on repayment plan?
You have to top up your gas meter at least once a week to tell your meter to take your repayment – even if you have credit on your meter.

So if you haven't been topping up, any missed repayment will go onto your meter as debt when you next top up.
How do I know how much I owe

How do I pay back my debt