How can I reattach the internal fan plug in?

Ouch Jessica, I'm afraid it's going to take some circuit board repair work to get your fan working again. The circuitry on a printed circuit board is made up of very thin strips of copper deposited onto a fiberglass board. The places where parts like your connector are soldered onto the board are called solder pads. Sometimes when a connector comes off the solder has given way, leaving the solder pads intact so the part can simply be soldered back on.

Unfortunately, it looks like the solder pads on yours came off the board. That means the connector can't just be sat back on the board and soldered in place, since the metal traces that it's supposed to be soldered to are gone now. Repairing the board is going to require tracing out what each pad connects to elsewhere on the board, gluing the connector back down, and running wires from those connection points to each pin on the connector.

So unless you have pretty good soldering skills, you may want to consider looking for a repair shop. This is a type of repair I've done myself at home, but I've been working in the electronics industry for a good 40 years, so I've picked up a few skills along the way.

Sorry I don't have better news, but the board can be repaired. If you are up to attempting it, we may be able to give you some help with figuring out where each pad needs to be wired to; I can make out a couple of the connections even on your picture as it is.

Good luck with your board; be sure to come back and let us know how it all turns out, or feel free to ask questions if anything isn't clear.