Hahaha! I had one of those. It's a time when you question your existence and what you have done with your life. Has it turned out the way you wanted it to? Those hopes and dreams you had in your twenties - did you achieve them? Or are you stuck in a humdrum existence, no excitement, a mortgage, 4 teenage kids, a job that's a drag?
I was one of the lucky ones. I'd paid off my mortgage, didn't have teenage kids. I quit my job and had a change. Got off the treadmill, bucked the system. First you have to realize you're in one.
For some people this period - often around your 40s and for some it happens early 50's - is really difficult. Heaps of emotional turmoil, agitation and even bitterness that you have headed down a path you didn't want in the first place.
If you are happy with your life you will not have trouble with this period. If you are dissatisfied then make changes. If you don't you may be laden with regret.