What Causes Delay In Periods?

Hi, my period was due today but I havent got it yet. Normally i get it 3 to 4 days before my pervious month date. I knew from my doctor that I have polycycstic ovaries but its not very serious since i get my periods every month and has asked me to just keep a watch on my weight which I have been. The delay in periods is causing stress. Could you pls recommend any medicne that stimulate my periods and i get in a day or two. Also, I am not pregnant since I have not been sexually active.