Unable to access the App Store on my iMac

I am suddenly not able to access the App Store on my brand-new iMac. I have been able to in the past and I always write down my password so I know that the password I first entered was correct but an error message came up saying that either my email address or my password was incorrect and to reset my password, although I knew 100%, that it was correct. So I reset the password with no effect and still the same problem. I have done this multiple times and still cannot access the App Store. I really don’t know what to do now as I wish to download some apps onto the Mac, but I cannot , find away of accessing the App Store at all. From what I have read on the forums it seems to be a common issue at the moment. Is there any body out there that can give me some advice apart from doing a hard reset which I am very reluctant to do?

[Re-Titled by Moderator]