It also sounds like you may have achieved a cultural victory. If you built libraries/temples/universities/cathedrals/wonders in all of your cities, you can get to 100,000 culture pretty quickly (consult your cultural advisor {F5} to see what your current culture is.) I used to get so frustrated by winning cultural victories (I'd be right in the middle of a war and a screen would pop up saying I had won) I would turn the cultural victory option off.
By default, you (or the AI!) can win the game in one of the following ways:
Destroy every other civilization (conquest victory)
Control at least 67% of the land area on the map (domination victory)
Get to 100,000 culture AND have at least twice as much as any other civ (cultural victory)
Get to 20,000 culture in one city (20k cultural victory)
Get elected Secretary-General of the UN (diplomatic victory)
Build all 10 parts of the spaceship and launch it (spaceship victory)
If you haven't achieved one of these victory conditions by 2050 (540 turns), the game will end and the player with the highest score wins.