I lost a blade

Yesterday, I started doing some mowing, and noticed that my Husqvarna 48" riding mower wasn't cutting a full "swath". I pulled the deflector up, and the R/H blade was Missing. A couple of days, prior, I used the rider to cut some tall. knee high weeds at the edge of the yard, and suspect that the blade flew off in that area. I walked all over that area, and couldn't find the blade....and I can't understand how something so Major could happen without noticing a big "bang" or some other...

I lost a blade