I Am 78 Y/o In Relative Good Helth. I Have

I am 78 y/o in relative good helth. I have had the flu for 3 weeks but feeling better. yesterday my legs simply gave out. The sensation feels like I have no leg under my knee. My legs are weak and have trouble gettin up from a sitting position. when I try to go downstirs i fall. Coming out of the docs office yesterday, I had a curb to navigate and fell. What in the worls is wrong with me?