I'm still fairly early in starting to learn 3d modelling, and I've been following a youtube tutorial step by step when I encountered this rendering issue.

The scene:

Render result:

I even switched to Mental Ray. Any idea how to fix this?

This could be a hundred things, considering that you are new it could even be that you just turned off the texture for that object when rendering.

First thing to test is the image, some codecs and images isn't fully supported by Maya; replace the texture with any other image you can find.

If the otter image renders then open Shop/Gimp and save the image as a .png or .jpg.

Second to test if it's the render properties on the material, change the material for a new one - a single color will do- and render.

If the material renders add the texture to it.

If even with a new material the object renders gray, delete it and replace it with a new object. Apply the red material and see if it renders.

If you are attempting to use one object with more than one texture study this tutorial:

Like most YouTube tutorials it isn't the most efficient way of doing it, however it is a good place to start.

A simple way of solving a multi texture material, is to not use them. One material one texture works best for games, so use more than one material on the object instead.