RMarkdown broken today after windows 10 reboot

I've been struggling with this for 6 weeks with intermittent success. After working with rmarkdown all morning on my W10 laptop, I shut down and restarted in another office - I do this everyday. When I restarted RStudio, rmarkdown stopped compiling to pdf. The HTML works fine. The gist of the error message was that pdflatex.exe can't be found. I'd had this issue 6 weeks ago after ~4 years of successful rmarkdown use every day and thought it was fixed. I'm using the basic .rmd template and all the defaults that RStudio wants. The specific error message is below. I've gone through several rounds of un/reinstalling rmarkdown, miktex, tinytex, etc. with appropriate restarts/reboots in between with no luck. I really like rmarkdown/bookdown and I want to keep using it but deadlines are approaching and I'm dead in the water. Can anyone please help? Is there a simple set of instructions on how to (re)install so this will work?
R 3.6.2, RStudio 1.2.5033 and the latest rmarkdown, tinytex (from github - installed w/o comment), and Miktex 2.9.

Error: Failed to compile Tester.tex. See https://yihui.org/tinytex/r/#debugging for debugging tips.
In addition: Warning messages:
1: In system2(..., stdout = if (use_file_stdout()) f1 else FALSE, stderr = f2) :
'CreateProcess' failed to run 'C:\Users\David Stevens\AppData\Local\Programs\MiKTeX 2.9\miktex\bin\x64\pdflatex.exe -halt-on-error -interaction=batchmode "Tester.tex"'
2: In system2("tlmgr", args, ...) :
'CreateProcess' failed to run 'C:\Users\David Stevens\AppData\Roaming\TinyTeX\bin\win32\tlmgr.bat update --all --self'
Execution halted

Condolences. I'm not on Windows, so I can only offer general guidance.

First, can you confirm that

'C:\Users\David Stevens\AppData\Local\Programs\MiKTeX 2.9\miktex\bin\x64\pdflatex.exe

exists, that it is executable freely, with whatever permissions RStudio is running under and, if possible that from powershell or whatever the current CLI that W10 uses that pdflatex.exe anything` responds?

And, as the cover of Hitchhiker's Guide to the Galaxy admonishes, don't panic! Worse come to worst, there's always the option of doing the work on RCloud instances or a provisioned EC2 instance. (Or asking friends for help!)

Let me know what you find. I'll be online for a couple of hours.

This just in: multiple pdflatex.exe leads to this error. And seems fixable in RStudio

Yes, it exists in the c:/users/David Stevens/AppData/Local/Programs/Miktex 2.9/miktex/bin/x64 folder. If I execute it from R (system('pdflatex.exe')) the return code is 127 with no error or warning message.

If I set 'wait=TRUE', the response is "> system('pdflatex.exe',wait=FALSE)
[1] 127
Warning message:
In system("pdflatex.exe", wait = FALSE) :
'CreateProcess' failed to run 'C:\Users\David Stevens\AppData\Local\Programs\MiKTeX 2.9\miktex\bin\x64\pdflatex.exe'"

If I execute it from the file system, a command window pops open with the text "This is pdfTeX, Version 3.14159265-2.6-1.40.20 (MiKTeX 2.9.7250 64-bit)
**" and that's all.

The mystery is that Ihad no problems for 4 years and now, suddenly, failure.

David

Technocrat - I searched my system and there's only one pdflatex.exe that I found. It's in the MikTex 2.9 ... folder. This folder is in the path.

David

Also, the solution offered by 'seems fixable in RStudio' followed by a recompile gave the same error.

Error: Failed to compile Tester.tex. See https://yihui.org/tinytex/r/#debugging for debugging tips.
In addition: Warning messages:
1: In system2(..., stdout = if (use_file_stdout()) f1 else FALSE, stderr = f2) :
'CreateProcess' failed to run 'C:\Users\David Stevens\AppData\Local\Programs\MiKTeX 2.9\miktex\bin\x64\pdflatex.exe -halt-on-error -interaction=batchmode "Tester.tex"'
2: In system2("tlmgr", args, ...) :
'CreateProcess' failed to run 'C:\Users\David Stevens\AppData\Roaming\TinyTeX\bin\win32\tlmgr.bat update --all --self'
Execution halted

Do you have these boxed checked in RStudio preference in both project and global?

It is checked in Global options. My project options doesn't offer that choice.