My Roomba 2.1 looks like this:
I bought this one used off of ebay.
It was working well until I tried to clean it.
I used a slightly damp rag to wipe off the dirt.
After that, it wouldn’t turn on for a while.
There was no response or light when I tried to press the “on” button.
So then I let it charge for a while on “home base.”
The the green “on” light did turn on after this, but when I pressed the “clean” button, it just makes an “ehh” sound.
It does not move- it just makes that one-beep sound.
I looked up that the “ehh” sound occurs when “Roomba’s Cliff Sensors are dirty or malfunctioning.”
So I cleaned out the sensors and internal workings thoroughly, but it did not help.
Then, I looked up that you can disable the cliff sensors by taping the shiny side of aluminum foil over the sensor areas.
I did this, but it still made the “ehh” sound.
Then I was irritated and just put the roomba back on the home base charger.
This morning I tried to start the roomba again, and it made a new 5-beep sound.
This looked up that this sound indicates that the “Roomba’s Cliff Sensors are obstructed.”
Well, of course they are obstructed because there is tin foil over them… So I guess the tin foil method for disabling cliff sensors does not work…? 
What do you think is wrong? How could I fix it?
Do you think the cliff sensors themselves are broken? …But if that was the case, how could the roomba detect that the cliff sensors were obstructed?
Please help me! Thank you SO much!!! 