Give more details as to model #. Anything changed by you?
Is filter clean? Checked for obstructions in venting? Any water leaks?
If it's a high efficiency unit, carefully check the rubber hose from the induced draft fan if clogged. Remove and blow through it. Other end connects to the pressure switch which may not be functioning properly.
Refer to Troubleshooting pdf chart chart here: (mid page)
1. Combustion fan/induced draft fan comes on
2. Pressure hose jumps, air pressure switch closes/clicks
3. Ignitor glows
4. Gas valve opens, burners light
5. Blower motor starts