that's a tough one.. sudden trouble might mean something got loose or broke.
After a failed attempt to start, examine the spark plug's tip. wet or dry? This should indicate if too much or too little fuel is being delivered. (dirt in the carb can cause either)
If the plug is dry, check fuel flow through filters and screens, etc.
If wet (aside from carburation trouble) there could be ignition trouble.. start off with the points .. file them clean if dirty.
While your in there check ignition timing.. it may have slipped..
And it's not unknown that the flywheel keyway key (woodruff key) has sheared ..
Check all nuts and bolts, especially around the carburetor and exhaust.. head bolts too. Any kinda air leaks can do it.
you wanna get lucky and find something obvious..