So the wizard is apparently hit with an attack. The wizard reacts using shield. Is that first attack automatically denied or does the +5AC come in and then the GM determines if the hit would have exceeded the new +5 AC? If the Wizards AC was 14 and the GM rolled a 15 with a +5 to hit for a 20. Would the reaction then fail?
It seems that the shield is intended to deflect the first attack completely.
Watching Critical Role it seems that Matt just assumes the 1st hit misses. He runs his own games and has house rules. I am not sure if this could be one of them.
After the reaction the AC is increased +5 and it seems that all Magic Missle attacks would also miss.
If the Wizards AC was 14 and the GM rolled a 15 with a +5 to hit for a 20. Would the reaction then fail?
Well... according to Mike Mearls: Wizards are expected to know the attack roll total before deciding to use the spell. So assuming the DM is announcing totals, a player using the spell has likely done the math already.
I typically don't tell my players what my rolls are, mostly because I use Roll20 and my enemy rolls are all GM whispered (juuust in case I need to fudge something to prevent a player getting obliterated in a not so fun way or whatever) however I know my players' AC's and I know their reactions; if something would be be blockable by Shield when my Sorcerer player is attacked, I tell him "You could block this with your Shield spell if you want" or something.
Many abilities allow you to add bonuses to attack or to your AC after you establish if you would hit or miss or be hit or missed. I would personality like to know an actual number but, I could see a DM telling you only that you will be hit and asking you what you want to try to do about it.