Ruud Silhouette II UGLH-07EAUER. The igniter lights but the burners never come on. The furnace then blows out cold air. What can be the problem?

Is there natural gas or propane available? Is the valve turned off. Did you pay the bill? I have been on many service calls where the gas company turned off and locked the gas supply at rhe regulator. It's not a stupid question. Those baxters are ruthless!

The answer above is correct but also inconclusive. After confirming the gas was not shut-off.
You need to check for dust collection. Take your vacuum and brush attachment and gently vacuum around the gas rails, sensors, and while you're in there might as well clean what you can. Then, if available, blow compressed air in and around the gas rails. Dust can create enough blockage to keep the igniter from igniting the gas/burner flames. This is an annual maintenance service that should not be skipped. Or you maybe trying to fix your furnace when you need it most. Lol

I found one recently where ants had clogged the gas orifices.