Engine will not crank over?

My car was totally fine when I got home from the store. When I woke up the next morning it wouldn’t start. I tried jumping it with two separate cars and two sets of jumper cables that didn’t work. I’ve also recently had my starter replaced so it’s not that. So, I removed my battery and took it to O'reilly's to have it checked out. They said the battery was only 2 years old and that it worked fine, they refilled and recharged it and I went back home to put it in, and it still wouldn’t start. As I’m securing the battery down, I noticed this wire with an inline fuse holder connected to it with a green 30-amp fuse inside with a pigtail end to it sticking out at me as if I forgot to connect back to the battery somehow but there wasn’t a place for it. As I’m trying to lightly pull on the wire to follow where it leads it slides completely out as if it weren’t connected to anything at all. The end to it honestly looks like it’s been clean cut. I did some research and found out that it supposedly controls the amount of power run through a circuit to protect specific lines. Here’s my question, is the wire that I found actually the reason why my car won’t start if so, where does it go and how do I put it back on? If that isn’t the reason could it be a different fuse in either of my fuse boxes that I need to check or changed if so, which one should I be checking?