No sound after mixdown

Hello everybody…
First, sorry for my poor English,I will try to use my best one…
I have a great problem with the mixdown option.
I have a project with 20 tracks, lots of vsti ( wavestate native, modwave native and Steinberg vsti) and some audio tracks.
All tracks go to my stereo out 1 and all is alright,I have sound and it’s funny.
So, after making a mixdown, all the sound of all my tracks disappear.
No vumeter level in my console, no sound, except the track of my mixdown.( Wave or MP3)
Is someone have the answer for that problem?
Thanks a lot
Serge