I've recently experienced a rash of events where maybe once a week on average mouse clicks have simply stopped working. The cursor tracks the mouse, but clicks just don't do anything.
After some weeks of irritation I eventually realised that I could avoid a reboot by restarting the Finder using the "Force Quit" feature (you can kill any app except Finder, for which the available action is "Relaunch").
"So how," I hear you asking, "do I do that when I can't click to bring up the Apple menu?" The answer is "With the keyboard. Simples!"
The Force Quit applet has a keyboard shortcut (Cmd-Option-ESC). Enter that key combination and the Force Quit dialogue appears. Use the arrow keys to navigate to select Finder. Enter triggers the relaunch, and a further Enter confirms that. In summary:
Navigate to Finder