The exact same thing happened to mine once, I just left it until it went kaput alltogether and they replaced the meter - I was uber skint at the time. I never ever got billed for the "free period" (it was a few years ago) But that might not be the case for you.
If you do end up getting billed for it, I'm sure you'd have to pay it back in installmenets through your key/card
We have a pay as you go electric meter. It hasn't run out for several weeks <months> I've finally got round to looking at it today The screen is totally blank So do I 1. Own up to electric company (usually use £25 per week) 2. Enjoy free electric 3. Ignore and hope it sorts itself out