I have a yamaha PSR 240 and one of the keys (E) won't make a sound. Everything else works perfectly except for that. I could not find a category for yamaha keyboards or musical instruments.

Yep, a key contact has got contaminated... You have to disassemble the unit to expose the keys.

You have to remove the offending key which is OFTEN tied to a bunch of keys. Under the keys you will find strips of silicon rubber with little domes sticking up. Under the domes are two contacts, either two black conductive rubber or sometimes a ring with a pill in the middle. These contact circuit traces with a black coating on the board under the keys. Clean the pills AND the circuit traces GENTLY with ONLY 99% isoprophyl alcohol. Reassemble the keys and the unit.

Be very careful handling, especially the ribbon cables between teh upper and lower halves of the unit.

Note there are TWO contacts per key which one closes before the other and that is how the key velocity is measured by the processor.