They say 90% of the fish are in 10% of the water, and the other 10% of the fish are in 90% of the water. If you're still allowed to fish it, get down there and fish it because 100% of the fish are now super concentrated in what remaining water is left.
As stated above, this could be done for numerous reasons. I know around here, most of the man made lakes with a dam get drawn down a fair bit every winter so they can maintain the dam. Also, because I live around the mountains, some lakes will get drawn down every so many years so they can dredge out the silt that gets washed into the lake. Over years, you would be surprised how a lake thats 30 feet deep becomes 20 feet deep because of the silt that gets washed in. They do this fairly often around here, and it usually takes the lake a while to get back to normal, but it's usually for the best in the long run.
Regardless, even if you're not allowed to fish, head down there. You might discover structure that you never knew was there, or you might see where the DNR is putting in artificial fish structure. Or you might find someone's old rod/reel or lures that can be salvaged.