Horizontal cracks like the ones in the picture in block basement walls might be an indication of foundation problems. Usually the first sign that the walls are bowing.
I recommend you to get a foundation repair specialist there as soon as possible because only a in-home structural assessment can give you a exact idea of how serious the problem is and what you can do to fix it.
Call 2 or 3 foundation repair companies in your area for a free inspection, get different opinions. It will cost you nothing to find out whether this is a serious problem.
This website has some information about foundation repair:
What Causes Basement Wall Failure
However, if you want to take an in depth look at the problem there is this nice free e-book that gives you all you need to know about foundations, in everyday language and with plenty of pictures