VeraCrypt is a popular open-source tool that uses the .hc extension for its encrypted volumes. These files act like "virtual disks" that require a password to mount and access.
If you meant with a lost key, you may need forensic recovery tools, not a generic article.
That said, I'll offer some general guidance on how to approach decrypting .hc files and highlight a few tools and methods that might be helpful. Please note that the effectiveness of these methods can depend on the encryption method used.
(a successor to TrueCrypt) or certain mobile VPN configurations like HTTP Custom 1. VeraCrypt (Standard Encryption Container)