Why is my wife of 12 years not interested in sex anymore?

Married 12+ years. We have 5 children ages 11 to 6. I am a entrepreneur and have had financial troubles over past year so I know that stresses a marriage. We love our children and that does take lots of energy. However my wife as an aerobics instructor is in great physical shape but everytime I need affection whether it be lovemaking or just simple hugs and kisses, she always says NO! She says she is to tired or is busy doing something.