What will happen to the IDG disconnection mechanism if we press and hold it for more than 3 secs?
(A320 training manual)

Pressing the IDG disconnect sends a current to a solenoid. A solenoid is "a cylindrical coil of wire acting as a magnet when carrying electric current."

When activated it pulls a pin that is holding another spring loaded pin that engages the disconnect mechanism.

Holding it for +3 seconds will overheat and thus destroy the solenoid and will require replacement to be able to use it again.

The disconnect mechanism requires a running engine. See here: What happens when disengaging the electricity generators on a B747?

So if it is not running, the next start may also damage the disconnecting mechanism.