Questions reguarding cooling system

[smdimjesus]

I did a massive update to my cooling system, and after it was all said and done, I was driving down the road and got an alert on the dash that said check coolant level. I pulled over and checked and it was empty and the engine was smoking. In a panic I tried to put more in and it went straight through it. I thought I blew up the reservoir or something but I found the transmission cooler detached at the top. I messed with the clip for the cooler a little bit and jammed it back in, drove home from where I pulled over and its so far so good. I'm trying to figure out if I could have lost transmission fluid this way. Also i'm curious as to how it got disconnected, maybe when I was replacing the reservoir. Also need to figure out how to have this not happen again.