What causes the battery to drain when the key is.

Please clarify "nothing seems to work". Do you mean nothing appears to be on that shouldn't be on when the ignition switch is off, or do you mean nothing works later because the battery is dead?

How long does this take to happen? Overnight or a month? Because of the many computer memory circuits that draw current all the time, the battery is only guaranteed to start the engine if it has been sitting for up to three weeks; less if it's an older battery.

There are dozens of computers and if any one of them fails to turn off, it will run the battery down. Aftermarket radios wired incorrectly can do it. A glove box light staying on will do it.

There is a special procedure needed now for measuring current drain because we have to contend with computers that take up to 20 minutes to go to "sleep mode". The procedure isn't complicated but it is time-consuming.