Furnaces built after 1990 have a tiny window where a light shows through. That light can flash a code to help you know what’s going on. If you’ve flipped the furnace switch off, then back on, note the sequence of the flashing light. Then open the furnace’s access panels. Inside one will be a key that tells you what the code means. That meaning will be useful information to tell a technician if the furnace still won’t start after you replace the panels.