I am posting this with the hope it helps someone avoid spending frustrating hour after hour trying to figure out why an Rmd project ran perfectly fine previously until suddenly the RStudio IDE stopped responding, all because of an errant space before a chunk back tick.
The first clue was the background shading of the chunk code and in-line text wasn't quite right but all the chunks seems to execute just fine from the beginning until it got near the end. Then I noticed that when the IDE stopped responding to chunk run commands the green progress bar showing the progress while running was missing, as were the little icons to run a chunk or all chunks above.
Searches of this forum and other RStudio help pages came up empty.
In turn, quitting RStudio, restarting my MacBook Pro running macOS Catalina 10.15.4, downloading the latest RStudio IDE for Mac Desktop (version 1.2.5033) and each time reloading my project and testing it only to see that when the RStudio IDE got most of they way through the chunks it would just stop responding without any error messages.
In desperation I started to reenter the code and discovered an errant space in front of one of the chunk back ticks. Removing it cleared everything up!
I was surprised that such a minor thing would trigger so many issues without an easy way to figure out what was going on by searching on-line or on these forums. Maybe the top FAQ could be "Issues with running Rmd chunks? - Check all back ticks"