If you have set a text style to have a text height of zero units and try to place an MTEXT box in your drawing, the box will not show.

However, you can select the MTEXT box because you know where it should be (use a crossing window to select it).

When you located the 'missing' MTEXT box, make sure it is still selected and look in the properties box associated with the entity. You will see that the character text height is set at By Style [which just happened to be set at zero in this example].

MTEXT

Remedy

Change character height away from zero (to 2.0) and your text will magically appear.

Fix it permanently

Use Format>Text styles and change the height value away from zero. 

MTEXT not working