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If the drain line extends down to ground level you can use your shop vacuum that also picks up water and connect the hose to the end of the drain line. This will pull the debris out of the pipe where it is plugged. This of course assumes that the drain in the attic does not have an air vent.
If the drains were properly installed there should be a clean out vent tee in the line right
However if you do not have either a clean out near the unit trap system, or ability to reach or even see the end of the drain pipe to use the vacuum then you need to cut the drain line and install a clean out tee into it.
Being sure your cutting the drain line, its the 3/4 inch plastic pipe for most cases, cut it out using a hack saw and simply glue the new plastic tee in. Be sure to extend the tee up a few inches and put a cap on the end. If your unit does not have a trap on the exit near the unit, you will need to install one. At that point I would suggest you hire a HVAC professional for this. As depending upon the system type and size there are several technical concerns about proper sizing that you would need to be aware of. And choosing the wrong type or size of trap can cause even worse backups of water.
There are really three reasons why your drain stopped working.
1. Bees have made a nest in the output side of the drain pipe and have prevented the water from getting out.
2. Your filter system is not doing its job. And the dust in the air has collected on the cooling fins inside the unit in the attic and has begun to fall off and plug the drain line. You need to get the coil cleaned and a better filter installed or changed more often as well to completely solve this issue.
3. You have no trap when the system should have one. And its not really a plug in the line at all. Traps prevent air from being pulled up the drain line when properly sized. Without the trap air flow goes up the drain line preventing the condensate water from draining out. On extended run times the water eventually overflows in the air handler in the attic causing leaks. As soon as the unit turns off the water can then drain out of the unit as it should.
My thinking is that the filter system has not be properly maintained and dust/lint has collected within the unit(s) in the attic causing your issue of not draining as it should. A cleaning using the vacuum or bottle brush will get things flowing again, but for only a short time or one season. By using a professional and having them come in, clean both the drain and the inside coil and condensate collection pan. You remove the risk of leak damage, mold development, and lower the cost of the operation and increase the life of the system you have.
Last note, If the pipe is not pitched away from the unit inside to the outside and water is sitting in the horz pipe from the unit, you need to support this better as the dust that does drain out will collect and plug the pipe along its length.