It might not work anymore so it was unplugged. The master of the house may not like using it so it was unplugged. It might not have a water panel inside so someone didn't want it turned on by mistake. If it is the type with a round drum that rotates in a pan of water never plug it back in. Those are breeding grounds for mold. The good kind have a 2 inch thick aluminum screen that water runs down.

Open it up and investigate the unit. Figure out how it works and if it works.

Its probably unplugged because its been that way over the summer when you should not use it.

We have a 90% condensing furnace. I just noticed the humidifier isnt even plugged in? why is that? I'm going to be a future hvac tech. It's all wired up and everything but the power plug isnt plugged in.