Transmission/torque converter

I was driving normally and everthing was working fine and I let my truck sit for about 2 hours and after I tried to drive I had completely lost 1st and 2nd gear. There was no loud bang or clunking noise and my oil looks perfectly fine, its a 4l80e trans with a 3600 converter and now I only had 3rd gear and overdrive. If you have any ideas on what could be the issue please let me know.