Blank Files after Lightroom Upgrade?

I updated to the new version of Lightroom Classic. Apparently there was some sort of bug/error and several of my files now have no images at all just a grey box. 
when I go to my original NEFs in View NXi all the Nefs are gone. The majority of my files are intact just for the months of June and August many are missing. 

I tried going back to the original catalogue and migrating that to the new LR Classic - same result. 

Has anyone else upgraded and had similar problems? I think my images are gone for good and I am left with tiffs only. I do not understand what happened.

That sounds horrible, do you not keep backups of the RAW files that you can go back to?

Knock wood; I upgraded both LR and PS yesterday and it went rather seamlessly. I did have to reinstall NIK - which seems to be working fine BTW - as well as PK Sharpener. Topaz Remask and Noiseware had to be manually transferred to the new plugin file. It took about 15 minutes to complete including transferring about 70,000 images into the new LR catalogue. I must add that I routinely back up and optimize the LR catalogue.

I did the upgrade and all seems to be working fine.

If you had the file name associated with grey box you could search for the file in windows explorer (assuming you're using window) . Just get one name and search your images drive, that should point you the right area to find the rest. I thought lightroom kept the raw file along with a database entry which described how it had been processed. - non destructive. So presumably it has lost track of where the files are? But you say that View Nx can't see them either.

You may be able to get them back with file recovery software if they reallly have gone. I which case you would want to limit activity on your image dive until you attempt recovery. Maybe check the recycle bin for files?

First a few questions that other member's  will need to know"

1. Your OS?
2. Your OS version
3. Lightroom Classic or Lightroom CC?
4. When you first  launched the new LR you did allow it to upgrade (or is that update) your catalog?

That being said. following assuming OS is Windows, slightly different if OS is MAC.

In LR, in the Library module, move your mouse over to the left, to the "Folders", right click on the folder (sub folder?) that the images are in (but LR showing just a gray box??) left click on "Show Explorer", when Explorer comes up and the folder in question is highlighted, double click on it as to enter it. Now, are the NEFs in question listed???

Thanks for the replies.

As stated in my original post I upgraded to LR Classic and the catalogue was upgraded to the new version too. I am on Win10 on a PC.

Yes I do have backups of the originals Nefs.

My workflow is as follows:

I view my files in ViewNX-i then delete the non keepers and transfer the files via ViewNX-i to my image harddrive.
From thoses I selected the files I want to work on and put them in a sub-folder named Reworked.
I then import the Reworked folder into LR.
I then Export as tiffs from LR to the same Reworked folder after doing my adjustments.
Then open in Photoshop and finish off as desired keeping a master psd or psb file as required subject to size and end use requirements.
I keep my files by year/month/subject.

There are only two months that are affected, June and August 2017. The June files I can find in Explorer but if I import them into LR again many of them are greyed out, just a file name and a grey field, not the same as the dull grey image you get when a file has already been imported.

The August files are very strange. I can only find one NEF file in each subject for the month and the rest are gone even when viewed via Explorer. They do not show up in Viewnx-I either.