I have a 2005 Nissan Xterra, with 55,000 miles

I have a 2005 Nissan Xterra, with 55,000 miles. About a year ago, in the summer time, I replaced the Fuel Level Sensor in the fuel tank and everything has been fine since, with about 5000 additional miles.
A few weeks back during the bitter cold (-10 degress) in New Hampshire, I started the vehicle and drove out of the unheated garage about 100 feet, when the dash lights came on and the engine quit. I found that the fuel line at the fuel pump had popped off. I replaced the little blue snap plastic piece (called a RETER-QUICK CON), and made sure it had a snug fit, and was careful about the electrical connection, as well.
I went to try to start the vehicle (now 20 degrees) and after 10 tries, the engine still does not fire. I know when I did the Fuel Level Sensor repair, the engine fired after the second time of cranking, so I believe my procedure is okay.
Some things I wonder, why the fuel line popped off:
1. Would a fuel line pop off due to pressure, if a fuel line was frozen?
2. Would a fuel line pop off due to pressure, if something at the engine fuel system was locked up due to cold?
Now to get things going again:
1. How can I check to see if fuel is getting to the engine?
2. How can I tell if the fuel pump is working?