Why cant i hit the notes that i hit like two months ago?

im 17 so it cant be puberty i went through that at the end of freshman year, but i use to be able to hit the high notes in boyfriend by big time rush, no i find that im tryna push for the the note more than singing it, and ive noticed that my singing voice seems to sound deeper than it use to my normal voice never changed, and i dnt feel like i have a sore throat. i sening tenor two and bass 1 in choir n i love singing tenor i dnt wanna go back down to bass again so what can i do to get my range back up and whats wrong