check for spark at the plug wires (careful if it has power your ass is going to get lit up wrap a towel around the screwdriver if you do it that way). if there is none check the msd box for power by probing the small red wire, which turns it on, and the main power lead. also check for a GOOD ground, they need it. if the box has both power and ground it is very possible that it has gone bad. the are known to do that.
i traced a non fire issue before a race on one of the cars i crew on and it turned out that the msd just went bad. he drove it into the pits shut it off and then for the next practice it wouldn't start back up. box just went bad. i put a new one in and off she went.