I started it about a week ago, and something came up on brewing day, now it looks like it will be a couple weeks before I can brew. Will it still be good? Flask has been sealed since, not opened at all.
I would leave it in whatever container you have it in as long as it can be sealed and put it in your fridge. A couple days before brew day, bring it up to room temp and make another starter. Pour off most of the liquid above the yeast from the starter and discard. Then, shwirl the bottom half with the yeast and some of the liquid around and add the yeast goodness to the new starter you've made. Then you'll have a ripping starter come brew day.
Be very careful about sanitation. Dunk everything in Iodophor or something, etc. Minimize exposure of the containers to air when open. Have fun!