[AD&D 2E] Rolling system ingame

hi all i'm new to roll20 :) me and a few friends recently joined up as we were all interested in picking up d&d.. so the other night a friend made a quick dungeon for us to play on... i should state we only have 2e rules.. but anyway last night i was attacking with my fighter and i used the roll button with my weopon in my journal against a enemy wearing no armour... and i roll a crit 0. when i hovered over the result it gave me a -20... did i do something wrong? :) i'm puzzled how it was possible when i have no minus modifiers with a 10+ to hit

Was this a roll button from the character sheet, or the dice roller dialog? If it was the character sheet, then I suspect the roll button is probably handling THAC0 for you.

it was off the character sheet yes :)

2E, fun! As Brian suggested, it sounds like the way that button is programmed, it's calculating THAC0, so the number showing is actually saying you hit that AC or worse. Sounds like you're first level, and you rolled a 20, so you hit AC 0.  Out of curiosity which 2E character sheet does your group use? There are 2 versions in the Community character sheets, the Simple version and the advanced one.

If it's the advanced sheet, what it is doing is telling you the AC struck by your attack. I can't speak for the simple sheet, but I'm fairly certain it's set up the same way. Those numbers sound accurate for a first level character.