Here’s my first-ever post in order to help you! =) Several days later; I’m surprised no-one responded to you. You haven’t indicated if you’re using a source-built engine or a prebuilt from the Epic Launcher. This section is for people building the engine from the source-code (available on Github).
The obvious first: months of work and no backup! I see you already understand your mistake, so I’ll instead suggest using source control (such as Git) in the future. Source control will let you go back and review (and revert as needed!) individual changes you have ‘committed’ in the past, without needing to keep and manage full copies as backups.
Next, if you haven’t already: make a backup now (a nice, full duplicate to be safe). You absolutely CAN make it much worse - especially with the suggestions I’m going to give you!
I only use Linux for development, so I cannot help you with getting the JsonQuery plugin to build. That said, it does sound like a dependency issue. The warnings aren’t always helpful. From memory, the engine may expect a very specific SDK sub-version in Windows but I may be wrong.
To recover your work (if it is only Blueprint and you haven’t actually done any C++ work), you can try to manually ‘revert’ it to a Blueprint-only project. You can do this by removing the project “Source” directory (which should only contain the empty class you made) and manually remove the generated modules entry from the “uproject” file (in any plaintext editor - should be one with the project name). The project should then be able to open as a Blueprint-only project just as it did before ‘converting’ it.
If the engine complains about a plugin being out of date, you can try disabling the problem plugin/s. If you’re using a prebuilt engine (which I’d guess you are, despite posting here, given the dependency error you have), perhaps see if someone would be kind enough to provide you with a prebuilt binary of the JsonQuery plugin for your Epic-prebuilt engine version.
Note that you could make everything worse by following my suggestions, so do make that full backup. It’s a bit bodgy, so wouldn’t suggest any of this if someone else more knowledgeable about Windows issues replied, but no-one else has. This should at least get you to where you were prior to trying to enable the JsonQuery plugin.