When the cooktop doesn't work at all, it may not have electrical power. Check the house circuit breaker for the cooktop and reset the breaker if tripped.
For an electric cooktop that plugs into an electrical outlet, make sure the power cord is plugged into the outlet. For a gas cooktop, make sure the power cord is plugged into the outlet. Check the electrical outlet by plugging in a lamp or small appliance into the electrical outlet to make sure it has power. If the outlet is dead, have an electrician repair the outlet.
If the burners on a gas cooktop aren't getting any gas when you open a burner valve, check the gas supply shut-off valve to make sure it's fully open. If the shut-off valve is fully open, you may need to have your gas supplier check the gas supply pressure. If gas supply pressure is okay, a failed pressure regulator on the cooktop may be preventing gas flow to the burners.
For an electric cooktop that uses an electronic control board to control the surface elements, a failed electronic control board also can prevent the entire cooktop from working.