What would you do in the following situtation:

You are approaching a non towered field, communicate right and all that
stuff, your on final, suddenly a cessna pulls up and starts it's take
off roll.

Also lets bring the same situation to a towered airport where the
controller tells you to go around while you are on final.

I am asking this because I am just wondering that at some point the
aircraft could collide (one climbing out, the other at pattern
altitude), and what the pilot going around should do...