start menu problem

Suddenly now when i click my start button on the left bottom side of my computer, the start menu pops up but the left hand side is totally black and when i place my mouse over all programs i cannot see or have access to my programs, i have to go to my desktop to click on the programs. The right side is ok, i can see my computer/ my music , my pictures files, run, search ,etc.
thanks for any help you can give me