Ok so I replaced the strings on my acoustic three weeks ago but since it was my first time changing strings I broke my high E string. I then found some two year old strings of the same gauge and used that high E. since I didnt want to buy a whole new set of strings. I was then tuning my guitar today and that high E string snapped out of no where.

So my question is after years of just sitting in my closet do the strings become weaker then brand new strings?
Yes, if they are sitting they may become rusty. That is what most likely happened to yours. You should be able to buy single strings at your local guitar shop.