After update OS X 10.9.3 MacBook Pro 2010, Restart suddenly
There are reports of Dropbox causing kernel panics on 10.9.3. If you have Dropbox installed, try quitting the dropbox agent. If the kernel panics cease, then wait for Dropbox to update their software.

First, click the dropbox icon in the menu bar, then click the gear icon in the bottom-right.


Then quit Dropbox from the popup menu.