I suppose that most people have encountered the elevator bug once or more times, most likely in the Maintenance Tunnels or the tunnel for 049. I have also suffered many times from this but since I don't want to flame around (as making games like this is pretty hard I guess) I tried around and found a few good hints to help you through this.

1.) you should NEVER save inside the tunnels or the elevator to avoid the chance of getting stuck, save when you're staying in front of the elevator or something
2.) if you've went down with the elevator and see that the switch is gone, you should first look if the other elevator works at least (there should always be 2)
3.) if you realize that all switches are gone, then you can either let yourself get caught by (this way you can get Maynard's code AND try to escape the Pocket Dimension for the achievement after the ending) OR
4.) if there are no switches and you don't want to mess around with larry, you can simply re-load the game and stay outside of the elevator and push the switch a few times, making the elevator go up and down. This works most of the time and gives you the switches in the tunnels back.
5.) if you want to make it easy you can also just use the console (F3) and activate noclip, but I personally don't like cheating and this way you also won't get the achievement in the ending (if that's important to you)

So to sum it up, save in front of the elevator and let the elevator drive up and down a few times. Or get caught by larry if you have no other choice, he will release you in another area.