We have a Mueller Climatrol furnace, model #137-115, and when I woke up about 6 hours ago I discovered that there was only cold air blowing through the vents. The pilot light is lit, but I have no clue where to go from here. Can anyone help?
Would like to resolve this asap, as it's Christmas, and have family coming by.

Check thermostat settings first. This may sound trivial, but it is also commonly overlooked.

Have thermostat set to 70, and it's currently below 50 actual. If you mean a thermostat on the furnace itself, have no clue where to look for it.

Is the selector set to heat? Fan switch set to auto? If so, then set selector to off, see if blower stops. Post results.

Yes, it is set to heat... and on auto. Was working fine last night, and we changed nothing.

Is the selector set to heat? Fan switch set to auto? If so, then set selector to off, see if blower stops. Post results.Did you get all of this? There is a point to it...

Do you have a fan/limit switch with a round metal dial on it?

Thank you all for you help. The problem has been resolved. There was apparently a fault within the thermostat. It turned the fans on, but not the heat. It has been replaced with a much more efficient one.

Glad you got it fixed, take care, and Merry Christmas.