One lucky Lotto player won the maximum £20m prize on Saturday night (August 6).
But, the ticket holder hasn't contacted Camelot to collect their winnings just yet.
The National Lottery's organiser urged everyone to check their numbers if they bought a ticket.
It means that Wednesday's jackpot will revert to £2m.
Camelot's Andy Carter said that anyone who thinks they've won should get in touch as soon as possible.
Saturday's winning Lotto numbers were 14, 15, 31, 41, 44, 45 and the bonus number was 20.
Lotto draws take place twice a week, on Saturdays and Wednesdays.
Most newsagents and supermarkets across the UK sell tickets from 8am to 11pm all year round.
But on draw day tickets can only be purchased up to an hour before.
The Thunderball jackpot can be worth up to £500,000, which is nothing compared to the main jackpot... but not many people would turn it down!
As well as changing the lives of the prize winners, the National Lottery funds projects all over the UK.
Since 2004, the Big Lottery Fund has distributed £9.4billion raised by the National Lottery to 130,000 "good causes" across the UK.
