gas furnace does not produce heat blows cold air?
the blower on a gas furnace is only supposed to com on when the heat exchanger is hot. look at your thermostat and make sure the fan in set to auto and not on if the fan is on auto and the blower is still running, your furnaces fan switch is stuck closed , you have an open limit switch, a welded fan relay, or possibly a popped flame roll out switch. call the gas company, or a HVAC guy. Possum, HVAC guy
depends on the type of furnace you have. Some use a pilot light. Others use sensors that will trip (like a breaker inside the unit). Some, like mine, use a gas sensor in there that will not allow the unit to fire if there is any air in the gas line. The gas valve may not be opening. Also, depending on how old the unit is, you can remove panels and check the circuit board for a LED that will flash codes to you that can be checked on the panel cover to help troubleshoot the system. I would call a HVAC man personally. I don't like messing around with gas too much.
90% afue furnaces can be quite complicated. It should have a diagnostic LED that will flash a code in the event of a failure condition and a key chart to tell you why it locked out. Sounds like a fail to light condition to me and could be an ignitor or spark issue. Best to have a pro look at it.
