Together Energy Retail has stopped trading, affecting 176,000 households.
Ofgem has now told Brits what to do if they're affected.
A statement on the Together website said: "We regret to inform you that the company will cease trading with immediate effect. We want to thank you sincerely for your custom over the past five years."
Customers of Together and its sub-brand Bristol Energy will now be contacted by a new supplier, which will be chosen by regulator Ofgem.
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Together also had one business customer.
Ofgem is advising Together customers to take a meter reading and wait until they are contacted by a new energy firm.
Experts say the collapse of another energy company means disruption for customers and rising prices in April.
Citizens Advice head of energy policy Gillian Cooper said: “Together Energy is the 27th energy supplier to have collapsed since August.
"As well as causing considerable disruption and confusion for customers, today's announcement will add to the £2.6billion bill consumers are already facing due to these failures.
“With the price cap expected to go up again in April and the cost of living soaring, many are already facing impossible choices between heating and eating."
Ofgem director of retail Neil Lawrence said: "Ofgem’s number one priority is to protect customers. We know this is a worrying time for many people and news of a supplier going out of business can be unsettling.
“I want to reassure affected customers that they do not need to worry, under our safety net we’ll make sure your energy supplies continue.
"Ofgem will choose a new supplier for you and while we are doing this our advice is to wait until we appoint a new supplier and do not switch in the meantime. You can rely on your energy supply as normal."
