undo randomly doesn't work??

Can someone explain why, sometimes, undo doesn’t work? I can’t see the pattern. Just now I accidentally hit x, return when everything was selected in Object mode, my whole project disappeared, and undo wouldn’t bring it back. Auto save saved me, but I would really like to know why this happened.

Hi, after reading the Blender Wiki and looking for an answer on the threads I am still stumped as to how “undo/redo” works in Blender. I’ve lost some good work because I have not understood how to do this. It’s not simple like photoshop undo.

Is it possible for someone to explain hoe undo/redo works in Blender in simple layman terms. Thanks.