geologists speculate that there have been times
in Earth's history when the planet experienced a
kind of Greenhouse Effect in reverse

that is, a runaway cycle in which Earth's snow and
ice reflected progressively more of the Sun's radia-
tion causing -

- temperatures to drop
- the further expansion of glaciers and snow cover
- the reflection of an even greater percentage of
solar radiation
- further drops in temperature . . . and so on

until the Earth became what Scientific American called a "Cosmic Snowball" hurtling through space

this theory is called Snowball Earth; and BBC Science & Nature describes it in this way,

There is a controversial theory that for millions of years the Earth was entirely smothered in ice, up to one kilometer thick. The temperature hovers around -40ºC everywhere, even in the tropics and the equator. If it did, then virtually nothing could survive this ferocious climate. There are some tantalizing geological clues that show this theory may be true but the problem is, the clues and the Snowball Earth theory defy the laws of nature.

For over fifty years a group of scientists has been trying to prove this incredible period of Earth history. Struggling against scepticism and disbelief, now finally the many mysteries have been solved and the scientific community is slowly coming around to the extraordinary idea not just of the dramatic freeze, but of an equally dramatic thaw. Scientists across the world are starting to believe that in the past the Earth froze over completely for ten million years... then warmed up rapidly about 600 million years ago. Almost all life was wiped out. But out of the freeze emerged the first complex creatures on Earth. Scientists now believe that the so-called Snowball Earth theory could hold the key to the evolution of complex life on this planet.

the video below, an excerpt from BBC Horizon Snow-
ball Earth, describes the theory in greater detail