Hiya!
Scenario: Party is proceeding down a wooded path at night with torches. Heavy foliage lines the path, which is 30 feet wide. Noise off the side of the path up ahead draws their attention. PC#1 goes up to check it out, is asked to roll Perception and fails. Crossbow bolts are fired out of the dark, heavy foliage and strike PC#1. PC#2 asks if she saw the spot from where the bolts were shot. DM asks for Perception roll and there is a success. PC#2 fires her bow into that exact spot, rolling a natural 20. Unbeknownst to anyone in the party, after firing the crossbow bolts, the two enemies were able to stealth away from that spot (rolled higher on Stealth than any of the party member's Passive Perception).
How do you, as DM, rule that Crit?
So, to rephrase...
PC's walking at night, can't see past torch light into woods. Hear noise and stop. PC1 investigates up to (assuming) edge of dim torch light...and then gets shot. PC2 asks to see if she noticed which direction the shots came from and succeeds. PC2 shoots an arrow at where she thinks the attacker(s) might be.
Ok...at this point, I'd ignore all normal 5e "rules". Why? There are no real rules to cover this exactly. There are hiding rules, stealth rules, cover rules, darkness/light rules, combat rules, etc. A LOT of different things are being dealt with. This is where I just say "You can't see where they are...could be 20' away...could be 120' away. And the torchlight is between you and them. And the bushes. And the trees. Hmmm...you could get luck though...go ahead and shoot. You need a natural 20 to hit".
The player rolls and gets a 20. At this point the whole "crit" thing kicks in. I'd ask to roll to hit again. If this roll would normally hit the crossbowman, then it's a Critical. If they miss, then it was just a normal hit.
This does several things. First it keep players engaged in stead of just throwing up their hands and saying "Well, I guess we run and hope we don't die...because it's impossible for us to hit them". Players feeling completely helpless because of the way a rule/rules work is almost never a good thing...in this case, it would be a "not good" thing (not ALWAYS, but usually). Second, it gives potential for those "lucky shots", or "million-to-one" type situations you read and see in movies...even if the chances are pretty much zero (e.g., movie "True Lies" where she drops the uzi down the stairs and it fires off randomly, killing a half dozen bad guys...'by chance'). Third, it reinforces the role and need of the DM as arbiter of the "campaign world", as opposed to him being a slave to the rules.
TL;DR So... in short, I'd say Natural 20 to hit, roll a "confirmation hit" to see if it is a Critical or just a normal hit.
^_^
Paul L. Ming