How does somebody lose there vision?

Can they ever get it back?

There are many causes:-
Macular Degeneration, Retinitis Pigmentosa, Usher Syndrome, Stargardt Disease, Best Disease, Choroideremia, Retinoschisis, Leber Congenital Amaurosis, Blue-Cone Monochromacy, Cone-Rod Dystrophy, Malattia Leventinese, Oguchi Disease, Rod-Cone Dystrophy, Refsum, Retinitis Punctata Alescens, Bardet-Biedl syndrome.
Causes of Blindness (http://www.blindness.org/visiondisorders/causes.asp)

You can even go blind through stress/ depression. As far as I'm aware no one knows the mechansism behind this.

The prognosis (whether you regain sight or not) depends on the cause and severity of the disease thst caused the blindness.

If you have your eyes covered up during the critial period of the develpoment of sight related neurons in the brain you will also go blind. This is irreversible.

It's a very big question! If you look at the site I posted and want anything explained just post up again. :)

EDIT-----------

I forgot cataracts, Diabetic retinopathy and Trachoma.

Wow- Thanks.

So if I'm stressed/depressed my vision can become worse?

I'll check out the site now.

Hysterical Blindness pretty rare, and it's usually quiet sudden, I wouldn't worry about it. Its most common with Post Traumatic Stress.

Are you worried about your sight? You should go see an optometrist or a doctor if you are.

I wera glasses.
But It was one of those things I always wodnered about.
The thought of me going blind scares me!