This is a really useful article if you use TrueCrypt or VeraCrypt, if you have encrypted part of a drive or an external drive then somehow lose your encryption key (this is called the volume header) - this could happen by reinstalling Windows.  You can create a copy of your volume header and store it online in Dropbox or something similar.

https://www.neowin.net/forum/topic/984006-guide-backup-truecrypt-disk-header/