I was looking to adjust some of the settings in TexShop and decided to reset my changes. I opened the 'Set Default Values' checkbox and clicked 'Regular' believing my changes would be reversed. Unfortunately it appears on installation MacTex had adjusted settings for my system and a message was displayed saying the path to the 'pdftex' command did not exist when I tried to typeset. After some investigating I found that one default path (/Library/TeX/texbin) was the one that had been set while another one (./usr/texbin) wasn't. I checked this other path and sure enough this second path existed on my system as an 'alias' (a mac feature that allows one folder to act as a 'portal' to another folder to programs that access it the alias as a folder). As such I changed my 'Path settings' '(pdf)Tex' path to './usr/texbin'. There is no longer a warning but upon pressing typeset nothing is printed in console and nothing happens. I thought maybe its because the file hasn't changed (not that this happened before) so I tried to edit the file and the program froze. I really just want to know how can I reset all the settings back to the way they were on install, not the default settings but my install settings (note, the installer of MacTex set these, not me).
I uninstalled TeXShop by deleting it from the Applications/TeX folder, and then I downloaded and reinstalled it. The paths were fixed and I was able to recompile without any issues. This process kept many of my settings, including my Recent Files list. Nice. This approach is probably a bit drastic (and likely has unintended consequences) but it worked for me in just a few minutes, and it let me avoid investigating and fixing the problem manually.
Note: I ran into the same problem by pressing 'Set Default Values'. In my case I was playing around with the "Source | Text and Background Colors" and "Source | Editor" settings in the TexShop -> Preferences menu. I didn't like my changes and wanted to revert them. I hadn't previously changed any of the other settings in the Preferences menu, so I figured that 'Set Default Values' would be relatively harmless. It was shocking and a bit frustrating to discover that I had ruined the paths to LaTeX and pdflatex. This is not the friendliest UI design in TexWorks, and perhaps some type of change could improve it.