My iPad cannot either download new applications anymore, or realize new purchases, but it's still able to update old applications. What does it mean???

Lots of information regarding my problem are already in the title:

My iPad cannot either download new applications anymore, or realize new purchases.

Once I enter your password, the downloading can't be done or, in the case of a purchase, the purchase fails.

But what is really surprising is that it is still possible to update my 'old' apps.

So my iPad is still able to connect to the internet, and able to download material.

What does this mean ???

I noticed that the problem appears every time after I entered my password (but the password I entered is recognized as being correct).

Please, is there anyone who could help me?

Thank you in advance.