It sounds like it got a big increase in the amount of direct sun? I don't think asparagus ferns (which are really asparagus, not ferns) like full, all-day sun. It may very well be too much. Also, having AC blowing directly on it would be very rough on the plant - very drying, and very cold. General rule of thumb is to keep (most) plants that are resting (i.e., not growing and perhaps with reduced foliage) dryer than those in active growth. Many plants that can tolerate or even enjoy standing in water when in active growth will rot if the wetness continues when they are not sucking up lots of water to support lots of leaves and new growth. If the combination of lots more (probably too much) sun, plus cold, dry air blowing on it constantly has shocked it to the point that most of its foliage is yellowing and dying, you could be in danger of rotting the plant. The stress it is undergoing could also make it more vulnerable to rot. Is there any soil left to it, enough to let it dry out a bit without exposing naked roots to air? From what you describe, I'd want to get the plant away from that AC vent, and kept it a bit dryer (if possible) until it starts to send up some new shoots. Good luck!

"Asparagus Fern" isn't a fern. It's a flowering plant that has small tubers underground. It's spread by seed, or by tubers from plants thrown away by people. In many places it's a declared weed. So, you can cut off the top of the plant, dig out the tubers and plant them in a pot. Any bits of tuber broken off and remaining in the ground will produce another plant. They can be very hard to get rid of.

Propagate using divisions. Asparagus is also easy to propagate by seed.

Unfortunately it's quite hardy here. As long as the ground doesn't freeze, it won't die. Looks like it needs a MUCH bigger pot. The roots make tubers that store water, so it doesn't need to be kept moist but those roots need room to do their thing. Should grow prolifically in bright shade, no direct sun needed unless you're at very high latitude. (IDK about the kind Dave has, too turned off by digging up 50-60 volunteers of the other to want to find out. They can't be pulled, they just break off.)