This issue is related to the accent character display on Mac. If this issue arises, press and hold a key without accent options (like 'q') for 3-5 seconds. This should restore the ability to type.
Alternatively, disabling the character accent menu on your Mac may remedy the issue permanently. You can do this by launching the Terminal on your Mac and entering in the following line:
defaults write -g ApplePressAndHoldEnabled -bool false
Then restart MATLAB.
This will change your settings so that holding down a key on the keyboard types that key repeatedly, instead of bringing up the accent menu.
