File system error (-2147219200)

This morning, I downloaded a few files to my computer and when I tried to open them, there was a long pause and then the message "File system error (-2147219200)". The computer, an ASUS laptop running Windows 10, had been working normally until then.

I ran "check" on both the C: and D: logical drives (they are the same physical drive) and all was well. I went to try to open a command prompt so that I could run "chkdsk /r /f" but the WindowsKey+X didn't produce any results let alone the menu offering me a command prompt that I was expecting. I tried to type cmd in the Cortana search box but couldn't type in there. I tried to reboot the computer but the Windows icon doesn't react when I click on it.

I began to think that I had some kind of malware so spent most of the day running a full MalwareBytes scan which apparently found nothing. (After over 5 hours it hadn't found any infections but then it rebooted and presumably ran the rootkit scans I'd asked for while I was eating dinner and then rebooted again. I'm not sure where to find its final report.)

I have no idea what's going on or why but I'd love to get that computer working normally again - I'm on a different one now - so that I can get back to the development work I've been wanting to do all day long.

Can anyone suggest a way to diagnose my problem definitively and solve it?

Really? Somebody at BSOD told me I was in the wrong forum and should be here. I copied my post over to here, as requested, and now you're giving me heck for doing what they asked?

As a matter of fact, I *have* had a BSOD (or maybe two) on that same computer this past summer but didn't do anything at the time; I was in the middle of something and wanted to get back to it so I just rebooted and it worked fine afterwards - until now. Unfortunately, I don't remember anything about those BSODs, especially the error codes, so they're not much help unless the system has archived useful information somewhere....