For Lydia specifically, the short version _might_ be to try the following command:

player.moveto A2C94

If that doesn't work in your game, or you are looking for a different NPC, then perhaps her reference ID is different for different people. If so, find her reference ID by saving your game as a text file. Do so by typing:

save "save-name-here" 1

If all goes well, the game will swap out and you should see a text editor filled with a text version of the save data. From here search for "Lydia". She'll show up twice in the file (normally), once with a reference of her base ID and one for her reference ID. The base ID should be 000A2C8E but you want the reference ID so you want the OTHER 8-digit hex code you'll find if you keep searching for her name in the file. Enter the player.moveto command and it should bring you to her. I recommend just making note of where she is, re-loading a save you didn't use the console on, and simply traveling via standard means to visit her. This way you won't suffer any possible side effects down the line.

This method is mainly useful for when you've told Lydia to stand still somewhere in the wilderness or a dungeon, fast traveled, then forgotten where you left her. If you save often you can just load a previous save but sometimes you need the big guns. Hope this helps someone.

Thanks a lot! Helped me find Shadowmere who was apparently too busy fighting a cave bear to follow me.

Excellent! I'm glad to hear this technique has broader uses than finding wayward housecarls!