Hi,
It would help to have more information. Currently, we can't advise much from what it is said.
! The log file hopefully contains the information to get MiKTeX going again:
! C:\Users\jeros\AppData\Local\MiKTeX\miktex\log\pdflatex.log
You should look into the log file.
! pdflatex: major issue: So far, you have not checked for MiKTeX updates.
It seems you may need an update.
See Ex_1.log for more info.
This log can also help.
If you are not bound to MikTex, you could also switch to TinyTex, a lightweight TexLive distrib that works well with Rmarkdown. See https://yihui.org/tinytex/