Without the password or a hint (do not post any info regarding it), this will be like finding a needle in a haystack.

The (.txt?) file where the passphrase was however, can be recovered depending on "how you've lost it".

• Did it get deleted? If so, you might need to use any file recovery software that can restore deleted files or formatted Disks. Some works on Flash drives as well.
• Did the flash drive or disk where it was stored got corrupted or damaged? As long as you've created the file on a PC and moved it there, you can just try to recover the deleted copy of it from the PC where it was created with again, any recovery software. If it was directly created to the flash drive, that's a bad news.