How much can a one really bad semester destroy my chances for MS in the top US/Canada Schools?
Several points. 
First of all - yes. You should definitely mention it. If there is a separate personal statement than that is the place. Often the first cutoff is mostly based on GPA and GRE. So mention it early, be concise, avoid any temptation of trying to gain sympathy (many have some difficulties along the way), and explain how you overcame that personal hardship.

It is what it is and you cannot change it. So chin up and forward march. Try to polish whatever you can polish. I you have a publication in a decent venue and good LOR's, it might cover for GPA issues any way. If your LORs are 'decent' and that is not modesty, I might start from there. 

Good luck!