have you tested the actual game? Use the command prompt and try to load the game that way.
Have you recently moved files? I moved my MameWah folder once and it didn't like it. You need to unzip it into a folder again if this is the case.
Do you have your rom directories set up correctly in MameWah? Try refreshing the games list. For that matter, do you have all the settings in Mamewah set up correctly, i.e. executable path, etc etc?
There are some MameWah tutorials around also, although I don't have the links offhand, sorry. There is also a program to help set it up for newer versions, but I am still using an old version (been meaning to upgrade).
Try that stuff first, and if you can, give us more info. Just saying its not working can mean anything. What are your settings?