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