PRACTICE, PRACTICE, PRACTICE. That's pretty much the short answer.
When my voice first changed, I was completely unable to sing in the high range anymore. But then after years of singing in choir, years of vocal warm-up exercises where you go up and down a scale slowly increasing the pitch by one note at a time, and doing that with a ton of different vowel sounds, eventually after months and months of work, it does start to stretch. I began my life with a changed voice as more of a baritone. But after YEARS of purposefully singing high notes, working on scales and stretching exercises, and still attempting to sing along to the same unchanged-voice songs that I used to be able to sing, even though they really didn't sound good anymore, eventually my range did once again stretch. And while I still can't sing the same soprano notes that I used to be able to, (damn it...) I can at least sing notes in the alto and first-tenor range comfortably again, while I had pretty much lost those notes when my voice first changed.
So, yeah. Practice, practice, practice, and above all, give it time! It's like learning to play a musical instrument. It takes a LOT of work to get the muscles to work in the way that you want them to, to build up the tone and the memory required. But just like the instrument, it's worth it in the end.