Macbook crashed during FileVault encryption.

See if you can boot into Safe Mode (it takes longer to boot). If you cannot boot into Safe Mode, then try Verbose Mode to see if there are any clues shown in the scrolling boot log. Usually the last entries will hold the clues since that is where it gets stuck.

If the above doesn't help or work, then you can erase the whole drive using Disk Utility. You may need to click on "View" and select "Show all devices" before the physical drive appears in the left pane of Disk Utility. Then you can reinstall macOS. This method will destroy all data on the drive, but I don't think you have any choice if Filevault is stuck.

Unfortunately if the computer is using the new APFS file system, there are no third party repair utilities available since Apple has not released the necessary AFPS documentation yet.

If the laptop was running High Sierra, then you need to boot into macOS 10.13+ in order to try and repair the drive & volume using Disk Utility First Aid. Try booting into Internet Recovery Mode using Command + Option + R which should boot the most current OS supported by the laptop. Sometimes Internet Recovery Mode doesn't load the newest OS, but the oldest one even when using the correct key combination. Or you can try creating a bootable macOS USB installer if you have another Mac.

https://support.apple.com/en-us/HT201372

If the laptop's drive is using the HFS+ file system (aka MacOS Extended), then the paid third party utility Disk Warrior is very good, but I don't know if it will work properly on a drive midway through the encryption process. Disk Warrior is unable to repair an APFS file system volume.

What version of macOS was running on the laptop? If it is High Sierra or newer, then Yosemite's Disk Utility won't be able to properly recognize or repair the newer volumes and file systems. AFAIK, there is no way to stop Filevault midway through the process.