Water level indicator using ULN2803

I'm not big in chemistry, but I believe that it has stopped working because of electrolysis. In other words your contacts have corroded to the point that the resistance is too high. You could prevent this by using AC, but then the circuit would be a lot more complicated. I would opt for capacitive sensors, Analog Devices have an interesting app note about Capacitive Liquid Sensors Or you can follow this Instructable

It's due to corrosion and electrolysis process mostly.

Go with some alternative thing like Float switche. Follow the link FLOAT SWITCH