why did my orchid flower dry and fall off?

Answer 1

perhaps it finished its flowering cycle. or it became too wet or too dry--orchids are real fussy about their climates. That is why most live in green houses or botanical gardens.

Answer 2

They bloom, mature, fade, and die. It's natural process, hon. Keep your orchid on a tray (drip pan) half full of small gravel and water - it adds humidity. It will bloom again for you next season.