The float may be jammed
Inside the tank is a float that protects the motor during wet pick up. It looks like a ping pong ball in a cage. This may be jammed in the up position and will shut off the motor suction. Unclip the lid of the tank and free up the float it, may be damp and sticky. Wash the assembly and leave to dry. Always make sure the cleaner is dried out following wet pick up before using to pick up dry. This will help stop the float from sticking shut.
Float mounted in the cage