How to recover BitLocker encrypted partition that is now ‘unallocated’/’free space’

I would try the bitlocker recovery tool

You can use this tool to help access encrypted data if the hard disk has been severely damaged. This tool can reconstruct critical parts of the drive and salvage recoverable data. A recovery password or recovery key is required to decrypt the data.

For Windows 7, Microsoft have some advice

The BitLocker Repair Tool (Repair-bde) is a command-line tool included with Windows 7 and Windows Server 2008 R2. This tool can be used to access encrypted data on a severely damaged hard disk if the drive was encrypted by using BitLocker Drive Encryption. Repair-bde can reconstruct critical parts of the drive and salvage recoverable data as long as a valid recovery password or recovery key is used to decrypt the data. The Repair-bde command-line tool is intended for use when the operating system does not start, or when you cannot start the BitLocker Recovery Console. If a drive has been physically damaged, it may not be recoverable.

This sort of experience does demonstrate the importance of making a daily backup of important data to an external hard disk or to another computer. It is probably worthwhile making occasional copies onto DVD and storing those in a separate building (in case of fire or burglary).