It probably means the fish is badly damaged and is dying. Please do a water change ASAP. Bad water is the commonest cause of death in goldfish. In order to keep fish sucessfully there are a few basic rules one MUST observe. They are :- 1 inch of fish needs at least 1 gallon of water. Every tank must have a permanently running 'cycled' filter. :- Every tank must have at least 50% of its water replaced every week..
They are ill, most likely, but no one can say exactly what they have; there are too many possibilities.
if it is not swimming and it lays on its side then it is dead.
Swim bladder disease? http://www.straightdope.com/columns/read/1688/if-a-goldfish-is-swimming-on-its-side-is-this-a-bad-sign
No!
Cause They Like Swimming Side Ways
If the goldfish is swimming in water, it is alive.
When their swimming upside down its usually a sign that their dying
Yes
it stops swimming
Gravity
Yes. It may have a hard time swimming for awhile until it gets used to it. Sorry, there is no cure.
Because your goldfish is dead.
IF you mean literally a goldfish, no. Just no. Otherwise provide information on what you mean by a "goldfish".