where the heck is my one burmese loach?

It's possible. If the gravel's coarse enough there'll be enough water in it for a fish to breathe, and as fish are cold-blooded they can go a long time without food (and that's even assuming there isn't anything to eat down there, which there might be depending on what you've put in the tank).

However, if it still hasn't come out and you start getting some fogginess in your tank, then it's possible the fish died while under the gravel and started to rot. If that happens, quickly get a tool and check under the gravel, because you want to get it out as soon as possible. Otherwise I wouldn't worry about it.

If you really that concerned you should clean your tank. The disturbance should scare it no matter where it is and it will start to swim around. It could have also blended into the colors of fake or live plants if it is dead. Loaches can also be known to jump out of the water. So if you're lid isn't properly put on you should check the premises outside of your tank.

it probably is in the gravel!