Citibank — my credit card cancelled without giving any reason.

I'm citibank credit card customer since more than 10 years and all of a sudden 4-5 days back my transaction got declined. When i called customer care they told my card account is closed during a internal review.

I asked for the reasons but they didn't told me any reason. It's very insulting that how can citibank suddenly close the account. Since so many years i always acted like a loyal customer and use to make timely payment and didn't had any issues. Kindly help to resolve the problem and reactivate the card.

Regards,