Draugr Wight Lord disarmed me, can't find my weapon

Disarm does not make your weapon disappear.

Unfortunately, Draugr usually live in the dark, and it can be very hard to find individual anythings in Skyrim, let alone in the dark crannies of its dungeons.

So your bow won't have simply vanished, and should be findable, if you want to devote the time and effort into locating it.

Realistically, you've got a few options:

• Load a previous save, fight the Draugr Wight again, and either avoid the disarm shout, or make sure you can pick up your bow immediately afterwards.
• Find your bow. If you want to do this, I'd suggest Candlelight / Magelight, and Unrelenting Force. Light up the area, and randomly try and blow everything around. You might knock your bow loose, as we humans are better at tracking moving things than picking out still objects.
• Mourn your bow and press on. There will be more Elven Bows down the road.

In case of not finding your weapon during the night, press T and wait until you get sunlight.

The best thing to do is give up and make or find a new weapon. This happened to me while doing the Staff of Magnus mission and I just used one of the enemies weapons. I lost a sword but I had a spare. Also, Lydia was killed so you were lucky just to have lost your bow.