I f I read the schematic right the op is only driving the opto led with 3.3, everything else driving the fet is 12v.
If the fet is hot without any pwm at the i/p it looks like it isnt switching off for some reason, maybe this is a layout issue, driver ics can go into oscillation if the layout isnt very good or there isnt enough decoupling.
Also 100r on the fet gate seems a little high, how did you get this value?, 10r seems more likely, check the 1n4148, if there has been bad ground bounce maybe you fried it, a uf4001 might be a better choice, also might be worth putting a similar diode across the motor connections to soak up bemf at the source, even if the fet has a diode built in.