Here in Louisville, KY we’ve had an enormous amount of rain so far this spring and therefore a lot of basements have been leaking. If your basement fills with enough water your furnace could get wet inside.
WARNING if your furnace (or any other electrical appliance for that matter) has been in water, do not turn it on. Turning it on is not an effective method to dry out the unit, in fact you most likely will do more harm than good.
If your unit has been in water first make sure the unit is completely powered down. Let the unit dry out, you can blow a box fan on it and/or use a dehumidifier to help speed up the drying process. If you turn on the unit before it is completely dry you run the risk of burning up components that are still wet from flooding.
If there is moisture remaining and you start the unit, you run the risk of electrocuting yourself or someone else or possibly cause a fire. Ideally you should really have the unit professionally inspected before you power back up.