This evening, I drove my lawn tractor into my garage to change the oil, spark plug, and air filter.

Everything went fine until I buttoned it back up and it won't start (it ran fine 20min earlier).

I attempt to start it normally with the attachment clutch disengage, the engine clutched fully depresses, and the engine choked. I get a constant whirring sound when I turn the key to start. Sounds kind of like a blender, but not as loud or rough.

Things I have tried:
-Cleaned old spark plug and reinstalled
-Removed Spark plug boot and verified firm contact with spark plug
-Verified Oil Level
-Checked battery voltage - 12.8V
-Jumped battery while starting
-Verified air filter installation

The lawn mower still makes the same noise when the key is turned.

I recently added gas yesterday from a container that was last filled about 3 - 4 months ago

Lawn Tractor specifics:
Craftsman Model 917.252531
15 HP OHV I/C Gold Briggs & Stratton
Model: 28N707
Type 0162 01
Code 9507174B

I know this thing is old, but it keeps going!

Remove the breather and put a little fuel in it and see what it does.
Luck

The whirring sounds makes me think the starter is not engaging the flywheel. Try putting your hand on the top of the engine where the wire mesh part is and turn it by hand. This will put the flywheel in a different position. If this doesn't help you will have to take off the plastic cover on the dipstick side of the engine so you can see the starter. Take a look at the gear that is on the top of the starter and see if it is missing any teeth.

I simply removed the plastic grill on top of the engine and turned it about 1/4 turn by hand (there was some resistance). Started up just fine after that! I guess it seized for some reason, maybe due to the oil change?

Thanks jsfocke!