I can’t open any of my Scapple files (V1.4.2.0). The desktop shortcut doesn’t work either. I haven’t updated anything (other than the scheduled Windows 11 updates). I’ve tried clearing cache, turning PC off & on etc. Scapple still denies me entry.
If I try to reinstall, will that destroy current Scapple files?
No. (Only your preferences.)
But at this point, did you try to simply launch Scapple? No file in particular, just Scapple?
If this works, no need to reinstall, the issue is not related to your installation.
Perhaps your antivirus doesn’t “like” it.
Sometimes people have related issues with Scrivener. I don’t recall this ever being a problem with Scapple, but you could disconnect from the internet and temporarily disable your antivirus, launch a Scapple file, see what happens.
Question:
Do your Scapple files still display the little Scapple icon?
Okay, well that was kind of interesting. I tried your suggestions - to no avail. I then tried to uninstall it but it wouldn’t let me. It kept telling me it was still running, so off to task manager to End Task. I tried multiple times, unsuccessfully. In a last ditch effort, I rebooted the PC and voila! It works (despite having rebooted it several times already). I guess I wasn’t holding my mouth right. 
I guess it felt threatened and decided to behave.