This sounds very similar to the problem that I am having with AutoCAD 2013 LT on Yosemite. Drawings just instantaneously disappear from the screen, AutoCAD doesn't 'crash' exactly, drop-down menus appear but are useless. The only way out is to force quit. When restarting the option to recover drawings appears but recovered drawings are very old and missing most (all?) of the changes since last save.