Gas stove cooktops often have a hissing sound associated with using them that you don't experience with electric units. The source is the gas flowing through the vents. If you don't hear the sound, it's not a problem as long as the burner will ignite. If you hear the hissing sound and your burner will not ignite, either the cooktop isn't getting gas or the burner vents are clogged.
Hissing Sound Source
When you use gas stove's cooktop burners, you often hear hissing or whooshing when you turn the knobs. This is the gas leaving the pipes that supply it to the unit and flowing through the air. The sound is normal and usually stops when you ignite the flame. You may hear it again if you turn the heating setting up as the unit releases more gas to increase the flame.
If you don't hear the hissing sound and your cooktop doesn't light, you may have a problem with the gas getting to your unit. Verify that the gas service is on in your building and there hasn't been an outage. Then check that the cooktop has a working gas line that's connected to an open gas valve. If all of the gas connections seem to be in working order, your burner vents may be clogged.
Sometimes your cooktop is getting gas through its line but the gas isn't getting through the burner vents. Over time, burner vents can become clogged with debris or food particles from normal use. Turn off the unit, and use a toothpick to clear out anything blocking the gas from flowing from the burner. Once your vents are clear, you should be able to use your burners again and hear the hissing sound when you turn them on.
Smelling the Gas Instead of Hearing It
Hearing your gas coming from your vents is normal, but smelling gas isn't. Smelling gas can indicate a gas leak, which can cause a fire or explosion. If you smell gas in the room, do not attempt to use your gas cooktop. Don't make any calls or turn on any light switches either. Instead, go to a safe place to call for gas service provider for a service call.