I have now experienced this problem two or three times. I am running AutoCad 2000 LT, and after editing a number of text items in the multiline edit, I can no longer select *any* entity on the drawing.
In fact, if I try a select all, it reports no objects available. Moreover, when in this state, zoom does not work. In any variation.
I have not been able to discover how to get out of this state. It has cost me the recreation of drawings, and is more than annoying.