Why does it say i cant download minecraft when i put in my credit number? And information?

So last night I was playing minecraft and the next day I log back on and it says demo mode. And it click why does it say demo mode? It says that I don't have a Mojang account along with my minecraft account. But the thing is that I do… So then I sign back into my account but it says the passwords wrong. So I decide to make a new minecraft account. After I type in all the credit card info. It says cannot purchase please try again something is wrong with your info. BUT NOTHING IS WRONG! Someone please help.

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