COA (Certificate Of Authenticity) is a sticker where the activation key is printed on. It's common on OEM versions of Windows.
Further reading:
https://www.microsoft.com/OEM/en/licensing/antipiracy/Pages/COA_hologram.aspx
Last time when you did this, did you also changed your MoBo?
Little insight how Windows and it's activation keys work (doesn't apply on Win 10).
When you install your Win and activate it with a key, Windows collects the hardware data (make and model of MoBo, CPU, storage devices, GPU etc) and sends them to the MS (Micro$oft). Since there are so many different PC component makers and models, it's almost impossible to have 2 PCs that have identical hardware. This way MS can keep tabs on the product keys they have released and also to prevent piracy.
For PC upgrade standpoint, you still can upgrade your PC, without changing the MoBo while replacing 1 or 2 components at a time (e.g add RAM, switch GPU or storage devices) without your Windows becoming deactivated. Though, there have been instances where everything was changed except the MoBo and Windows demanded a new activation key.
Since Windows activation key will be tied with the MoBo make and model, replacing the MoBo with different one, while keeping the rest of the system same will require you to insert a new activation key while your old activation key is invalid. Though, there have been instances when replacing only the MoBo with the exact same make and model hasn't provoked Windows to demand a new activation key.
The above is true with OEM versions of Windows. With retail versions of Windows things are a bit trickier. You can re-use your retail Windows key if you 1st deactivate your Windows on one PC and then reactivate it on completely different PC by using the very same key. Even with retail version of Windows, you can't run 2 activated Windows on 2 different PCs at the same time. If you need to do this, you'd be better off looking towards Enterprise edition of Windows that can be activated on several different PCs with the one single key.