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Your problem is not unique but it very seldom occurs. Let me give you a little background so that you can better understand the situation. My answer is based on the assumption that you are attempting to load a Microsoft product.
The usual situation:. Customers have taken in their systems along with all the software received when purchasing their system. The customer picks up the system and viola' no problems whatsoever until they need to download an update or even a template used with MS Office. Suddenly their system is non-genuine. The customer's computer was not installed with their software but a blocked version of a product key. Also the software could work for several weeks, months or even a year or more then suddenly becomes blocked.
Volume licensing (VLK) are purchased/issued to large corporations, governments, and schools for example in order to make software installation easier for the mass computers located within their areas. In turn if the key becomes compromised then the government, school etc requests Microsoft to block that key and reissue another key.
Product key not accepted. Whenever you are get an error about the key being invalid, the usual reasons are at least one character is typed incorrectly or you have a misprinted key. The latter is very rare, though it can happen.
I hate the font that they use for the key - some characters are miserable to discern. J and I look all too much alike, as do O and 0 (zero is the skinny one).
If you are still encountering a problem with activation, you have about 2 weeks to get activated, so I'd go ahead and open an email request to Microsoft Support, explaining the problem. If you can provide a digital image - scan or photo - of the key to the tech that responds, they can validate the key for you. This link will take you there: Microsoft Support
Have no fear: A Windows product key will not expire.
All the best