How can i sing those high notes i use too do so easily?

Ok so I am in choir but the problem is I cant sing thoose high notes anymore. I am a tenor. I also want too add that I feel like if I have a "blocked" throat everytime I go into thoose high notes. My vocal range in a B flat and an A flat. Can someone help me please because I have a Pops concert and a Advanced Group tryouts so I need help fast.