What model furnace? Does the blower come on but no heat? If so, it could be the sail switch. Blower does not come on? If so that's a different set of potential problems including a bad circuit board.
Most are remarkably simple in design and operation and there's only 2-3 things that ever go wrong with them. If you can see the circuit board and it's a newer generation furnace, it will have LEDs on it that flash an error code. You can then look up the code in the manual. If you can get access to them, they're normally pretty simple to fix once you figure out what the problem is.
2016 6.7 CTD 2500 BIG HORN MEGA CAB
2013 Forest River 3001W Windjammer
Equilizer Hitch
Honda EU2000
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