Going directly to the source to correct the problem
Basically, that plausible deniablity that everyone loves to quote is springing into action. If you're going to encrypt drives, you must in advance do everything to backup your keys, store your passwords safely and backup volume headers, among other things.
Per TrueCrypt on recovering a deleted volume.
The standard volume header uses the first 512 bytes of the TrueCrypt
container. It contains the master keys needed to decrypt the volume.
If the header gets corrupted or the container reformatted, TrueCrypt
will display Incorrect password or not a TrueCrypt volume.. Using a
backup of the volume header is the only possibility to recover the
Prepare for disaster in order to ensure success is the word of the day. If you didn't back the volume header up previous to the disaster, you aren't going to recover the volume. It's part of the safety mechanism if my reading on TrueCrypt is correct.
Various other iterations of damage/corruption recovery are covered and the use of TestDisk to recover from partition table overwrites, TrueCrypt Rescue Disk for recovery of encrypted information.
I think the first operation before doing anything would be to image the disk so you can make more than one attempt at recovery.