I had/have the same experience with my 8100.
I'm living currently in the philippines and there tend to be a lot of moister in the air if its not on the tables from the beer bottles
It stopped working twice now. The first time I opened it up, couldnt find anything, put it back together and it was working fine again after a day or so.
The second time I had exactly the same problem. All keys wont work anymore, so opened up again. Inspected the keypad a bit closer this time and I noticed that was moister under the bubbles of the keypad. With my finger i pushed out all the moister and wipped it with some dry cloth. Used the hair dryer for a while and put it back together ...
It has been working fine ever since ....
I did bought a 'sock' after those incidents though. Just to keep the moister out of the unit ...
I bed when you open up your unit and you with your finger lets say from the # key up to the 3 key you probably will see some fluents under the plastic ... Thats what you need to get out.
I need to say that 8100 has not been really made for easy repairs with regards to water damage!