Data decryption
Start by analysing the encryption.
Keep the original files and error message. The format, signs of encryption, available keys and backups determine whether decryption is possible.
Record the file type, the application that created it and the exact error. Note any changed extensions and whether a password, recovery key or earlier copy exists. Do not publish keys or passwords. Work only with data you own or are authorised to recover.
If the storage device is healthy, make a separate working copy. Do not rename or overwrite your only original. Correctly implemented encryption may be impossible to reverse without a key; a backup can offer a separate recovery path.