i have buy a CS6 Master collection before,but i have reset my computer yesterday.i forget to deactivate the product, so i can't activate the product now to use the full product?

You are allowed to have two activated installations, so if you already had two activations in use then you cannot actiovate a thrid unless you deactivate one of the other two.  If you only had one activation used up then you should still be able to activate a second installation.

You should contact Adobe support and have them deactivate the original one that you forgot to.

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