Show us what 'you' expect will be the final schematic for this project.Make sure you show all your connections.
EDIT:An IRL3705N would also be a good MOSFET you could use.Always use the devices data sheet to confirm lead identification.
OTOH you have quite likely simply blown the regulator.Define "fried"?Does it accept an upload of the "blink" test program when disconnected from your circuit and connected by the USB to the PC?
I could test the "fried" arduino with the new one as a tutorial I have seen online as last resort or do you have a better recommendation?
Indeed... miswire the MOSFET and you can get 12V into your Arduino pin. That would explain the symptoms.
Give it a ceremonial burial and grab a new one.Or just toss it in the trash can and grab a new, if you're not into ceremonial burials.Anyway, don't bother too much with the broken one. These boards are cheap, not worth trying to fix it.