Electrical system has gotten hot

When I first got in here after noticing the smell, the bottom white wire was practically broke off - it may have been completely off, but it came apart easily. I reattached it to the bar and tightened it down. The next wire up is a big neutral wire for the AC and it's badly discolored (I tried to just get some tape over it). When I went to loosen that bolt to try and remove the wire the whole bar twisted loose from the black insulating mounting plate, so that's still loose.

I am planning to replace this whole bar and plate tomorrow...but I'm still not sure why things got so hot to begin with???