Are you referring to the email service? I have the same problem, started 12 hours ago. I can't receive iCloud emails on my iPhone or Mac anymore. It keeps on telling me to re-enter my password or, on the iPhone, informs me my password is wrong.
Just go on www.icloud.com and sign it. Check if you can use the address and calendar functions (most likely you can). Then click on "email". Most likely it'll produce some error message and give you the option to send it to Apple.
If that's the case it's not a problem with your configuration but just Apple's mail server having issues. The only thing you can do is to wait and see......