what happens if I play Elnuk Herd from hidden targeting another friendly unit in my battlefield, then the target unit is bounced. how does Elnuk Herd resolve

No — the +3 Might is lost, and Elnuk Herd still recycles itself, so you end up with neither.

Sequence: the +3 target was locked in when the play effect went on the chain (you had to pick a friendly unit at the battlefield you played Elnuk to — Rule 811.1.d.3). If that unit is bounced in reaction, it's no longer on the board, so the buff instruction mistargets and is simply ignored; it does not get to re-choose and land on Elnuk instead (Rule 355.15 — choices can't be changed).

The recycle part survives. It's a separate instruction that only checks Elnuk's own status ("if I was played from face down"), not the buffed unit — it isn't a linked instruction to the buff (Rule 359.3.e.14), so the buff being ignored doesn't drag it down with it (contrast the "its controller draws 2" pattern in 359.3.e.12's Hidden Blade example). Elnuk is still on the board and the reference is intact, so it goes to the bottom of your Main Deck.

Net result: nothing is left at the battlefield and no Might was handed out — this is the opposite failure mode from FAQ #12771, where Elnuk itself left the board and the trigger did nothing at all.