One instrument in my score will not sound, any help is really appreciated!

Hello,
I have a score which consists of two trumpets (two separate scores), french horn, trombone, tuba and piano.
All voices were playing with sound until I wanted to mute the piano. I successfully muted the piano but now one of the trumpets will not sound at all. Furthermore, the piano is now pianissimo even after I unmuted it.
I have tried everything in the mixer, making sure no one voice is still muted or on solo. I have checked the volume in the mixer, and they all show at exactly the same level, and also velocity.
One thing I do not understand is if I have two trumpets each with their own stave, then why in the mixer is their only one panel for both? Is there one mixing? Perhaps this is the problem... As I said before, in the beginning all voices were sounding.
For clarity: when I click on a note from the trumpet it sounds, but in playback it does not at all.
Anyway, I would really appreciate any advice, help... anything as this is an assignment. Maybe someone has had this problem before and knows how to solve it?

Many Thanks! :)