We will be able to reset your Help to Buy: ISA with all the savings you had in at the time that the bonus was claimed. To start this process visit your local branch. The bonus you received will be returned to the scheme administrator, you will be able to claim this bonus again in future when you find another home.
If Covid-19 was the reason your house sale fell through (up to 30 June 2020) you have 15 months (rather than the normal 12) to restore your Help to Buy: ISA from the date you originally closed the account and your conveyancer should give you a Purchase Failure Notice stating 15 months.