The bottom drops out; the bottom falls out; the bottom fell out (of something)

The bottom drops out; the bottom falls out

A collapse occurs.

• The bottom dropped out of the steel market.
• When they lost the game, the bottom fell out of their hopes to make the playoffs.

This term alludes to collapsing deeper than the very lowest point, or bottom. [First half of 1600s]

... http://www.answers.com/topic/the-bottom-drops-out

the bottom fell out (of something)

something suddenly lost value.

• When the bottom fell out of the real estate market, a lot of people lost a lot of money.
  
  Usage notes: usually used in the past tense

.... http://idioms.thefreedictionary.com/the+bottom+fell+out