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There is a tiny set screw on the side of the knob. Make sure this is tight, and the knob is not just spinning around and around.
There is also a small rod inside -- that the focuser attaches to. If tightening the setscrew does not fix it, you will need to remove the focuser and see if it is detached from the rod. Make sure the scope is tilted BACK with the front of the scope UP a little or the mirror will slide to the front and possibly break.
This web site has lots of repair pictures-- http://www.weasner.com/etx/menu.html
The Meade ETX90 does NOT have small lens! It has a large curved lens in the front. and a holder for the eyepiece in the back (top). There is also a camera port in the back, and a flip lever that sends the light either straight back to a camera, or UP to the eyepiece holder.
There is an INTERNAL secondary mirror mounted on the back of the front large lens. If this fell off INSIDE, you will need to remove the front lens to fix it. NOT an easy fix as the optical alignment can get messed up when you do this.
Here is the manual for an ETX90, you should read it completely.