A telescope eyepiece focuses the faint light of stars, planets and other distant objects captured through the lenses contained in the barrel of the telescope. There are several reasons for the eyepiece not functioning, or a lack of visible images.

Lens Cap Off?

Check the front of the telescope for a lens cap, which protects the glass from scratches and other damage. You may not have yet removed it.

Weather Conditions

There may be cloud cover that is blocking astronomical observation. Even a light mist or fog can scatter the light from astronomical objects, making it impossible to achieve a sharp focus.

Wrong Eyepiece

If you have bought an eyepiece separately from the telescope, the eyepiece may not have the proper specifications or barrel size to use with your telescope. Standard barrel size is 1.25 inches.

Improper Magnification

You may be using an improper magnification for the object you are attempting to view. High-magnification eyepieces are used only for the moon and planets. Mid-range is for brighter stars, galaxies and nebulas, while low-magnification (with the widest field of view) is used for faint objects. The higher the magnification, generally, the poorer the image.

Poor Eyepiece Quality

You may be using a cheap eyepiece that cannot achieve clear focus across the entire field of view. Premium eyepieces are more complex and carefully designed mechanisms which allow clearer and deeper focus.

Mirror Damage

The mirrors used as objectives within the telescope barrel may be damaged or fogged. Check the seals of the fastenings on the telescope itself, or bring the instrument to the shop where you bought it for servicing.

Source:

Ten Questions about Telescope Eyepieces

Telescope Bluebook