Since it is a standard interior AC unit, the situation is different. Here are some thoughts ...
1. Again, the condensate line may be plugged. Routine maintenance blows the crud out of these lines. Here is a good general discussion:
Here is a DIY way to clear the line. You can make his attachment from parts available at Home Depot:
2. The catch pan might have rusted out (if it is not plastic). Check it and the connection point of the condensate line to the pan.
3. The condensate sump pump could have failed. The condensate line runs from the catch pan to a sump pump with a float ball to actuate the pump. With age, these do fail. Try to manually trip the pump. If it kicks on, this is probably not the issue. If it fails, you can obtain a replacement online or through Grainger.com
4. Again, the refrigerant line might be missing cork tape on the outside of the unit. Since it is very cold, it can condense water from the air and drip a lot. Have seen this cause leakages in the wall big time from this simple problem.