What is wrong with my voice?!?

I'm a singer in both my school's concert choir and barbershop quartet (yeah its dorky but fun). As a tenor, my head voice is just as important as my chest voice. So I've been sick for a week, and now I finally don't feel sick anymore. However, when I try to sing in my head voice nothing but a barely audible hint of a note escapes. Whats going on??