I think you have a refrigerant leak. The smaller line is normally filled with liquid (compressed) refrigerant when the system is running. Because it should be under pressure, it should stay liquid until it gets inside of the evaporator coil that's in the air stream (probably) above the furnace in the duct. The larger line contains the gas going back to the compressor after it has essentially boiled in the evaporator. Since it takes heat to boil things, it takes that heat out of the air going by, which cools the air.
So, if you don't have enough refrigerant in the system, the compressor can't produce the liquid, and then it can't boil, so then it can't cool. What little liquid it is compressing the gas is evaporating in the line, cooling it, before it gets inside.
Turn the thing off, as running the compressor without enough in it will ruin it. Find the leak, fix it, and if the compressor isn't shot, you'll be okay. It could be fairly inexpensive, or it could cost a bunch if the compressor is shot. Depends on where the leak is, how easy it is to fix, and if it has run long enough, low enough, to affect the compressor.