You’ll need to add enough credit to cover the energy you’ve used since your balance reached £0, plus £10 (depending on how you’re topping up) to get your meter started again. Remember, if you ran out of credit at the start of a bank holiday weekend, you could have used 3 - 4 days’ worth of energy over the weekend, that you’ll now need to pay for. As soon as you’re back to a positive balance, you’ll have your energy supply back.