Probably in the Yellow Pages for heating service, or your gas company.
Could be anything from a closed valve, to a thermostat/wiring problem,
to a stuck solenoid valve.
Probably time for regular service, anyhow.
It's probably the solenoid next to the gas inlet pipe. You can check
that easily by using an ohm meter to check for continuity in the
solenoid. You'll probably end up repacing the whole gas valve and it
might be time to consider a new furnace.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.