Sometimes, if you turn your body whilst lifting something, or when twisting out of a car (in a tight parking spot), say, you may get an intense pain in the lower back, which is very painful and prevents you from easily standing upright. That's regularly what's meant by your back's "gone" or "gone again".
I know, it's happened to me in the past.
Actually, the remedy, given to me by a physiotherapist, was quite simple, but can only normally be done indoors:
Lie face down, flat on the floor.
Do a "press-up", but keeping your hips on the floor and slowly trying to roll your body upwards and backwards towards you heels, until you are at elbow height, with your upper arms horizontal and pointing away from the body.
Repeat 10 times, relaxing absolutely completely, flat on your face between each press-up.
Then repeat the same, but with arms straight out, below you.
It's cured me many times in the past, but there is a catch:
Don't bend over forwards (supposedly for 4 days afterwards).
The first time I tried the cure it worked so successfully, I thought I could try out bending forwards. Never again!
A "bad back", is one that is susceptible to such problems.
Hope this helps explain it (and maybe even cure it, if you are unlucky enough to experience the problem).