The simple answer is that the energy company the energy switched to, will work hard to get it sorted out for you as quickly as possible. In a bit more detail:
You may want to advise the old supplier and new supplier that you think you might have been switched by mistake.
If you raise an Erroneous Transfer with us, we can work with our Energy Partner to resolve this on your behalf.
The switch back to the old supplier will usually take around 6-8 weeks.
The old supplier will bill for the energy used while our Energy Partner get the supply back to them, just as if nothing had happened.