The unit stopped producing pure water.
Over time, you may observe that your RO system takes longer to produce water. A 100gpd system should produce 4.16gph under optimum conditions. (When the source water is colder in the winter months, production rates will drop significantly. 50° F will reduce a membrane's rating by 50%; a 100gpd membrane will only produce 50gpd until the water warms up again.) If your RO unit no longer produces any water, if it only drips out, if it takes hours to produce one or two gallons, the membrane could be the cause. However, before you resort to a membrane replacement, check these first:
Disconnect the tubing after the membrane (prior to the DI) to see if water comes out of the product line, to assure the DI section isn't the problem.
1) If the flow restrictor has fallen out or was lost, water will only run out the waste line. The flow restrictor is an important fitting, inserted fully into the waste line prior to being pressed into the John Guest fitting on the housing. If you are missing the flow restrictor, try to find it. It may have dropped out when you were replacing the membrane and/or disconnecting the tubing.
If you don't have a flow restrictor, a temporary solution is to install a ball valve on the waste line. Close the valve to apply back pressure, which should start producing pure water. Through trial and error, you can adjust the ball valve until you reach a 4:1 ratio. Using two 2-liter bottles, you will want the waste water bottle to be full to the top, and the pure water bottle should be 25% full. Readjust until you achieve this correct volume. You can order a replacement flow restrictor here.
2) If the DI cartridge is installed upside down, no water can exit the good line. There is an arrow on the side of the DI to show you which way is up, which is also the direction the water should flow. Remove the DI cartridge, point it in the correct direction and reinstall.