Can you decompile Unity WebGL builds?

I doubt there is any tool out there. Unlike most other platforms a WebGL build does not contain your code in a .NET assembly. Your code is cross-compiled into (web) javascript. Other platforms like windows standalone, Android and even webplayer build contain your code as a compiled .NET assembly which can be decompiled once extracted. However WebGL as well as iOS (and maybe other platforms as well) don't use .NET assemblies at runtime. If you build for iOS your code is converted into an xcode project.

What does "crashed" actually mean? Did your HDD fail? I never had a "crash" where i lost a whole project. Sometimes you might loose a single file that was just written when it crashed.

Unless you try to get your hands on other peoples code i recommend you do more backups in the future.