Cubase not recognizing file locations on Saving, eating proj

RUNNING: WIN 7, CUBASE 8.5, KONTAKT 5, VEP PRO,RME 400, AKAI MPK88, 5 2TB 7500 DRIVES, 64 GB

Hi!
Tonight I was blocked in Cubase from resaving a project with a message that stated
“CUBASE PRO
The project could not be saved because the system cannot find the file specified.
The previous version of the Project has been left unchanged.
O.K.”
I retried 3 more times with the same result.
I then closed Cubase, reopened it and went back to the most recent version of the project. This time I got this message

" Invalid project file-(followed by the name of the file itself)."
O.K.?

I started this project last year, and have multiple saved versions of it, particularly over the last 3 days. I have not moved folders, only changed the title of the saved version to indicate what as done to it. Exactly the same way that I’ve been saving projects in Cubase since version # 5. I had saved it 4 times earlier today.

I then opened another project and tried to save it, and got the same response, both with the"cannot find file specified" message with the save attempt, and again with attempting to reopen this 2nd project and again being blocked from this second project with the same "Invalid project file " message. I am not going to open up anything else. At this point I’ve just lost 3&1/2 weeks of work.

I’ve checked through the system, the drives are all as they were this morning, the “lost” projects at issue here are sitting in there folders.
I’ve had no crashes, not brought in any new software, was only using the V.I.'s that I 've been using in the projects, and have used for years now.
We’ve had no power outs.
I just went from tweaking a section to having Cubase eat, now, two projects.

Can someone tell me what in the H is happening, and how to fix this -fast please.
I have just sold the commission and have a looming deadline for completion.
Thanks!
g.c

So I couldn’t sleep so I came back to the studio, booted up the computer (I shut down nightly-weather issues) and everything is now working just as its supposed to.
But the issue did happen, and I’d like to know what it was. All that comes to my mind is that the project drive may be going out. I’ll replace it right away as a precaution. But is there anything that could be going on that comes to anyone else an idea for me to look for?
Any input is appreciated!
Thanks!
Now I can go to bed.
g.c.

Hello - I had same or similiar problem today: I did it to myself by enabling Windows Defender security setting to block User folder updates.
Its under Start, Settings, Updates & Security, Open Windows Defender, Virus & threat protection, Virus & threat protection settings:
Then under the heading: ‘Controlled folder access’ -it’s set to On. Then from there clicked on 'Allow an app through Controlled folder access" -and then added the path to the Cubase exe file. (i.e. C:\Program Files\Steinberg\Cubase LE AI Elements 9.5\Cubase LE AI Elements 9.5.exe) - - hope this helps!!
jj1