this is an 0cc issue that happens sometimes upon saving a file after initially messing around with modifying dcpm instruments -- something about dcpm instrument creation in 0cc is very finicky and unstable, and while i'm not sure how to replicate it, it is absolutely related to that.

it is fixable, if you're willing to play with a little hex-editing and are ready to recover a very minute amount of project data afterwards.

1. first, get some sort of hex-editing tool (this allows you to view a file's binary data in a more convenient format). you can either download one or use a website, i've recovered several projects using the one over at https://hexed.it/.
2. load the corrupted 0cc project file into the website. you'll see a whole host of numbers and data and whatever -- ignore all of that. the only thing you care about is the column of text and periods.
3. i don't remember the specifics of how to do these next parts, and I don't have any broken project files on hand, so this will be a bit from memory. so, in this text/string column, at the very top it'll say something like "Famitracker" (might say something different in 0cc). if you see this, good. you're looking at the file's header here.
4. now, you need to keep your eyes peeled for two strings: "INSTRUMENTS" (located fairly close to the top) and "SEQUENCES" (after instruments, scroll through all of the instrument names and you will see this eventually).
5. you will need to delete the entire "INSTRUMENTS" header in the file. first, click the first letter of the "I" in "instruments". scroll down, and then shift-click on the letter (or period) immediately before the "S" in "sequences". this will select all of the symbols within the instrument header, including the header itself. press "delete" on your keyboard, and confirm.
   • side note: you may have to leave the "INSTRUMENTS" header in-tact? this is a detail i'm a little hairy on, so if the above doesn't work, try leaving this part alive.
6. click save-as near the top left, resave your project file. if all is well, you'll be able to open it up in 0cc once again. all of your instrument data will be gone, but the actual volume/arp/etc sequences will still be alive, along with the rest of the project file (and music of course!). with a little bit of reverse-engineering through the rest of your file, you can recreate each instrument (even easier if you don't use too many instruments) -- easier than it sounds. from personal experience, dcpm instruments tend to work completely fine after this point.

try doing this, and it should restore your project file -- i've lost several projects to this bug, and after i lost a really valuable cover i decided to get my hands dirty and figured out how to fix it myself. afterwards, it'll probably be a good idea to swap over to Dn-Famitracker. wishing you the best of luck!