I'm both a trombonist in a couple bands and a bass vocalist in an acappella group. I find that when I play trombone for an extended period of time, then switch to singing, I can't access the bottom major third or so of my vocal range (though my high range feels great). This remains the case for the next couple hours, even if I warm up my voice really well. Is there anything I can do to either stop this from happening or become able to get the low notes back faster?
I'm not a singer etc - but I got interested in a special technique of very low pitch singing and have done quite a lot of voice exercises lately.
So my first thought would be that playing your instrument strains your muscles in one way or another - and my experience is that doing growling ("vocal fry") for a while restores low notes in your voice (possibly by relaxing the many muscles involved).
So I'd try to take a comparatively low note - in your tired and limited voice - and then do the opposite to tensing or trying to produce a full volume sound - relax the cords more and more, and you'll fall into a growl (an American will call it "vocal fry"). Then growl in a gentle way - like a purring cat (feline purring is an exact equivalent of this sound). This "turns on" your false cords right above the real ones and somehow relaxes the whole machinery and makes low voice notes again possible - at least in my experience.
// The special technique I mentioned is called "subharmonic singing" - that is when your cords sustain a note one octave above the sound your listeners hear, due to the mechanism of false cords, which , when properly added to the base sound, oscillate at 1/2 frequency.
So every one of us has one more register below his bass register of about one octave in size.//
My guess- as a singer and a didgeridoo player- is that there is probably nothing you can do about it. The vocal chords vibrate in sympathy with tones you play on instruments, and get fatigued as though you were singing. All you can do is plan accordingly.