We’ll set your Help to Buy: ISA back up with the savings you had in at the point you claimed the bonus, but you'll need to visit your local branch to start the process. The bonus will return to the scheme administrator but can be claimed again when you find another home.
If your house sale has fallen through due to Covid-19 (up to 30 June 2020) you will have 15 months (rather than the normal 12) to reinstate your Help to Buy: ISA from the date you first closed the account and your conveyancer should give you a Purchase Failure Notice stating 15 months.