Till date, I have experienced four different situations in which Google ads does not display. Out of them in three of the four cases, all the ads are not shown in a page and in one case, only some adsense unit is not shown.
In case that if only one adsense ads are not shown, then check how many adsense units are added in a blog because in a page, no more than 3 ad unit can be displayed. This can be the case. The other case may be that ads which are newly made are not displayed until they are 10 minutes old.
Other three cases may be when
1. Your site is blocked: In this case just your site is blocked but not your account. Google send message when any of your site is blocked.
2.Your Account is blocked:In this case all your adsense account is blocked and you have no access to the account.
3.Google is temporary down: In this case, no ads are shown in your site because of Google database problem.
So now you can easily distinguish between problem and there is nothing to worry about when you find what real problem is.