I posted a solution to this problem for Windows, based on deleting the Appdata/Arduino folder, but now I believe I found the real issue:
Window has always had a problem with the limitation of depth of directory ramification + filename.
So when Arduino IDE tries to open a .ino file which exceeds this (meaning, too many folders within folder to get to the file + long filename) it fails, so does not open.
To make it worse, Arduino IDE always opens with the last code you saved, even if you're just clicking the .exe file. So if your last .ino file saved exceeded the limit, Arduino IDE wont open.
So I solved this by renaming this last file which caused the problem to "a.ino" and it's folder to "a" which made it possible to open the Arduino IDE again.
Uninstalling Arduino, deleting the Appdata/Arduino folder and reinstalling will make Arduino IDE forget which was the last file so it will open, on a blank code, but as soon as you try to open another file which exceed windows depth of directory + filename, Arduino will fail again. So avoid the too-many-folders-with-long-names issue.
Hope this helps!!