My heating worked all winter, but the first time I fired up the AC, no air comes out of any register on AC or heat. The AC uni is on outside, and air is blowing up from the bottom of the furnace. Any ideas?

Are you saying that the furnace fan is blowing but no air is coming out the registers? This would sound like a clogged filter to me.

You might also check for a frozen evaporator as described here.

Best of luck,
WallyH