Why Mathematica does not respond after the first activity?

Hi,
I have a MacBook Pro with 2.5 GHz Intel Core i5 with 10.8.5 OS X and I downloaded the latest version of Mathematica.
I do one calculation of any sort and then Mathematica goes in “running…” mode undefinitely. If I restart Mathematica, it works but only for the first activity.

I

Have you removed and then reinstalled Mathematica? The behavior you are observing is not common. There really is not much else that one can say to advise you aside from suggesting that you directly contact Wolfram Technical Support (some members of which are on this forum) with your problem and license information.

From Mathematica crashes on the first run :

My Tech Support reflex is to suggest resetting initialization files by holding down the SHIFT
and OPTION keys as Mathematica launches. (See http://support.wolfram.com/kb/3274.)

An alternative is to delete the folder Library/Mathematica/FrontEnd/ under your Home
directory, but that is by default hidden.

A diagnostic test is to try running the Mathematica Kernel by itself. In a Finder window:
     go to the /Applications directory,
     Control-click (or right-click) on the Mathematica app,
     choose “Show Package Contents” in the pop-up menu,
     in the next window open the Contents folder, then the MacOS folder.
     Double-click the MathKernel icon and a standalone kernel window will open.
This may show an error message explaining the problem, or it may run fine.
Either result is useful information.

In rare circumstances, once activation is complete the Front End opens without
problems when it stumbled before.

The Technical Support group (support@wolfram.com or support@wolfram.co.uk)
is the most direct place to go with cannot-run problems. Please include your
License Number or Activation Key.

Less likely but possible, you can try turning off the Suggestions Bar (also known as Predictive Interface) in the Preferences.
(Uncheck the bottom line under the Interface tab. See below.) Do this before the first evaluation.