I live in a two story house with a septic system. The downstairs bathroom suddenly started draining all the water out of the bowl after a complete flush cycle. I tried filling the bowl after it has drained same result no water in bowl after 5-10 minutes. All other drains and toilets work normally.

It sounds like you are having a vent problem, when you use other fixtures in your home your water closet (toilet) empties out because the vents are looking for air.

You may have a vent problem or even a crack in the toilet bowl. Manufacturers also sometimes find small holes or leaks in the tiolet in places you can't see. They put a small patch of clay on the leak and it sometimes comes loose years later. If the toilet empties every time in just a few minutes it's a crack or leak and not a vent. If you want to test further take the toilet off the floor and set it in the garage or basement. Slowly pour a bucket of water into the bowel until it comes just short of normal full line. If the toilet drains again its definitely a crack or bad patch, neither of which can be fixed by you. Eljer Patriot toilet 131-2120 two pice $98.