this happened to me and i discovered that the arial font file had been disabled. first check to see if
Arial.ttf has been moved to the disabled fonts directory. from the terminal:
ls /Library/Fonts\ Disabled
if so, move it back to the active fonts directory.
sudo mv /Library/Fonts\ Disabled/Arial.ttf /Library/Fonts
log out, log in, then open the ‘Font Book’ application. in my case, arial was present before hand, but it was the Microsoft version stored in
/Library/fonts/Microsoft/. Font Book may now show a yellow triangle next to the font name, indicating that multiple copies of the font exist. highlight the font name and chose the
Resolve Duplicates command from the
Edit menu. this should disable the Microsoft copy, which you can confirm by selecting one of the arial type faces (click the expand triangle next to the font name), right-click on the one labelled
Off, and choose
Reveal in Finder, which should open a window to Microsoft fonts directory.