I have no definative answers for this, sorry, but, you could try a factory reset (see sticky above)..
I know that several people are having problems with adding a second story. I'm not sure, as I have never had this problem, but, I get the impression that it is linked to EP's which I don't own. There is quite a long thread about it - something about crashing when adding a second story, finding, and reading that may help.
It may help if you tell us which EP's you have. Are you building on a foundation, or with a second or third story? Have you removed all CC? Have you recently installed a new EP? Are you fully updated? All this information may help others with more knowledge to isolate the problem.