When you spill a liquid on an electronic device, immediately power it down and take the battery out. In this case you can't really get the battery out so you'll have to settle with just powering it down. And I mean hold the lock button until you get the power down screen and power it down, don't just lock it.

The danger is that whatever liquid you spilled will short something that shouldn't be shorted. And it's obviously shorting one of the two pins in the headphone jack to the ground pin, that's why it thinks there is something plugged in. If it just got into the headphone jack, you'll probably be OK. If it's just a little bit, it probably didn't get anywhere else. If it were just water, you'd just leave it unpowered for a day or so and, if you can, take it apart and lay it out on a towel to dry. Coffee has little bits of coffee bean in it though, and those stay around. They will probably be OK as soon as the water dries up, assuming there's just a little bit in the headphone jack.

In other words, power it down and wait a day or so, you'll know then if you have a real problem.