I need help from u guys!

My stereo in my 2001 bmw x5 want give any sound.
Sometimes it can work for several weeks and suddenly it will not work, the sound goes away for some reson.

The stereo is still on and i got all the functions exept for the nav and sound, is this the display that is messing or is it in the trunk?

What can i do? I hate to drive without sound and would like to fix this a.s.a.p

/Thank´s!