completely screwed up?

i just attempted to make my first batch of homebrew stout and i believe i may have screwed it up completely. it started at 1.050 gravity and is supposed to be at 1.012 - 1.016 final gravity and it has stayed at 1.031 gravity over the past two-three weeks. i went back to the store and got some more yeast and put it in, but it still hasn't changed. i didn't even see any activity in the airlock when i put in the fresh yeast. so i'm wondering what i should do. should i go ahead and bottle it or is there something more that i can try adding for it to become the correct gravity? or is it now useless and completely screwed up? please help!

Why don't you first post your recipe...We can help you better.

post recipe.

DO NOT BOTTLE! if you bottle, I'd wager $1,000 you end up with exploding bottles. When I bottled a stout at 1.024, it blew up after 4 weeks in my closet. lost all 5 gallons.
Do not be impatient like I was!

it is my understanding that if you are going to pitch a second yeast they should be active first, so you make a starter with them, rather than just pitch

What temperature are you fermenting at?