When you put it away 4 years ago, did you put it in "Shut-down" mode?
In DC the Quantum Engineer has a shut-down button. In DCC one of the Function buttons will do it, usually F9. It goes thru a series of sounds, finishing with the engineer closing the door and locking up. Kinda neat, until you forget about what you did.
To recover you'll need the Quantum Engineer box hooked into the DC power (then push the "Start-up" button) or a DCC system that can send the Start-up function (usually F6).
Alternative would be to do a reset. The reed switch was a option for the Revolution, because the reed switch could fail in the closed position, which upset customers because it would reset at every power application. If it doesn't say "Reset" when you apply power, that's a good thing (you don't have a stuck reed switch).
Since we're talking steam here, the decoder is probably in the tender along with the speaker and it's associated magnet, not a good location for the reed switch. So if a reed switch is present it will probably be in the loco.
The Revolution manual says "Turnoff the power. Place the Magic Wand over the reed switch area, apply power and leave the wand in place until you hear the word "Reset" . Your locomotive is now reset to original factory defaults" You can try several places around the cab and boiler, hunting for the switch. A reset takes several seconds, so don't be impatient.
If that fails, the manual says "If you don 't have a reed switch installed; locate the pink wire" ... "and the black wire with a white stripe and simply touch the ends together as you apply power per the instructions above." Those are the wires that would be connected to a reed switch if i were installed. On the Revolution-U decoder these two wires come out of a connector on one end of the board. They are the last two positions on the connector that has two violet wires that go to the speaker on the opposite end.
Hope this helps.