This is entirely fixable!
Open your protected deck in any text editor and look for a line near the top of the file that reads
locked:1
Delete that line, save the file, and the deck should be "unprotected" again.
Also, for the record, all .html exported files can be imported into Decker just like .deck files- the only difference is that .html exports have a short preamble and a big suffix containing the web-decker runtime.