Hey trans specialists, I had a transmission cooler line pop off and empty the transmission (4l60e). I towed it home and fixed the cooler line, then refilled it. It drove great for a few miles and I let it sit. Then later that night, I drove it again to keep checking on it. That drive it had no third gear. It slips as if in neutral. I have checked the fluid level and added a bit more and it's not gone away. I don't see how it could fail that fast.
What can/should be done/checked?