When this happens, leave the ignition in the run position, pull off and ground your coil wire (to watch for spark), and grab the white wire on the msd.
Tap it to ground. Every time you break the ground connection, (by picking up the white wire) you should have a spark come from the coil wire.
If you don't, you either don't have power to the MSD box (main) or (switched), or a bad ground to the msd.
if it does spark, then your issue isn't with the MSD, it would probably be with the pickup in the dizzy, the coil going bad, or problems with associated wiring. (wiring from the dizzy chaffed and shorting or the like)
Coincidentally, if you read the owners manual, you can use the white wire for theft prevention. With it grounded the MSD won't fire.
edited slightly for clarity
93 Ranger f/s D44/9" spooled waiting on tax returns for 36" Iroks, jonesin for a f/s Bronco!
"Get up on this wisconsin death trip!" -static-x