Is this on Paypal? I was having this problem a few days ago, and contacted Paypal since it won't go away for a while. Turns out they block certain payments and processes if it is flagged suspicious.
Are you using a proxy of some sort? In my case, it was because I was using my mobile internet at the time (with ISP gateway proxy). As soon as my internet was restored and was able to login to Paypal with my usual connection, my payments went through without problems.
They say it's a security measure in place to protect users from suspicious transactions (or the like), and the reason would be on a case-to-case basis. Might be best to contact support if you can't figure it out.