Your character(s) almost died and is now unreachable? Fallen through elevators or even textures? There is one thing you can try.
It is a rare glitch.
Nonetheless it can happen that one or multiple of your characters fall through platforms, almost die and then stabilise. As a result they might become unreachable.
The problem here is: how to venture forth if you cannot gather your party?
One solution might be to “lethally liberate” your character with an attack from that situation and have them revived by Withers at camp. But what if you cannot reach them at all?
Use a Short Rest
That will “revive” your fallen character and make them usable again. In that state you can now use Waypoint teleportation via the map menu (press default “M”) to escape that predicament.
What if no more Short Rests are available?
In that case ungroup your party and have one character sent to Withers to respec into a Bard.
At level 2 Bards gain an ability which has the same effect as a Short Rest.
With that ability continue as described above and you should be good again.
Thanks for reading and happy adventuring!