How can I unblock my credit card?

You need to quickly call your bank. The reason it was blocked is because they thought someone stole your card (or your card details) so they're basically trying to protect you. If you contact them quickly enough, it'll be re-activated within the same day (assuming it's not in the weekend). However, going forward, you really should let them know before you travel, so it won't happen again.

That really depends on what you mean. If you're traveling, and you found your card to be denied, it was blocked for security reasons. So you can just call the issuer and let them know it was you who made the purchase. Also, in the future, you can just notify them that you'll be traveling, and you won't have the problem.
If you aren't travelling, it's possible your card was maxed out or defective. In the first case, you need to pay some of your balance off. In the second case, a good idea would be to call the issuer and ask for a replacement card.
Another reason might be that your card was blocked due to suspicion of fraud. Call them and find out.