Help me decompile my .jar file?

I'm making a minecraft mod, and somehow my entire source code got wiped off my PC's files (i checked my projects folder, it's erased from there too), but I still have the completed project as a .jar file. I want to edit that exact .jar file, but I have zero means of doing so, nor how to do it.

I'm super fresh new to this, and I see no possible way to edit the .jar file I spent weeks on after I lost my entire source code. When I try to open the .jar file with IntelliJ IDEA, all it keeps telling me is “(file name).jar is unsupported binary file”. I just want my source code back so I can continue working on my mod… Is it telling me that even the .jar file is destroyed? Is it a total loss? Is all my hard work destroyed forever? Do I need to start from the very beginning all over again?