A couple of questions.
When you say characters what exactly do you mean? moving characters, (barrels, jumpman/Mario, etc..) or ladders, girders, etc.. Or all characters?
Can you take a pic of your problem and post it?
Does the game play normally? (besides the missing characters) sounds, music etc..
A generic check would be to check your connectors on the game board (PCB).
Check the two gray ribbon cables (reseat them)
and finally reseat all the PROMs on the video board.
I would suggest that you do these "checks" one at a time the turn the power on between each "check" that way you can determine what the actual problem was. (It may be useful later if it happens again).
If that does not solve your problem then post a pic and perhaps someone can help you to determine the actual source of your problem.
Greg