This can happen due to a software or hardware upgrade or for no apparent reason. The mysqld.exe service crashes each time when you launch it, a solution for this :

Close the application that executes mysqld (For example, WampServer ...)
Start> Run
Run the command: services.msc
Right click on the entry "mysqld"> Properties
Connection tab> Check "Allow service to interact with desktop"
Apply Changes
Now you should be able to restart the service.