Make sure you're connected to the internet, and just click Activate in the settings app, if it lets you. If there's no Activate button, choose Troubleshoot. You should be able to enter the Windows 7 key, or if you have the option, choose "I changed hardware" or something like that, and select the PC model from the list. If you were signed in with a MS account before, that should reactivate it.
If that fails, just go to MS Support (live chat). They're pretty good at getting you reactivated. Make sure you have the Win7 key or other proof-of-purchase ready.
Also, if you have a UEFI system (basically any PC or motherboard less than 5 years old), you hould go into your BIOS, and make sure that Secure Boot is enabled, Fast Boot is enabled, and Legacy Support (sometimes CSM/Compatibility Support Module) is disabled. These can cause activation issues if set improperly.
Also, to (re)install Windows, use the official Media Creation Tool, available at
https://www.microsoft.com/en-ca/software-download/windows10.
Using other tools will likely not work, at best they are likely to set up the partitions improperly (not critical, but can cause some issues), at worst they're not installing a genuine copy of Windows. The official tool is always the best way to go.
If you were using a different tool, I would highly recommend redoing it with the official one. Make sure to check those three BIOS settings first, then create a USB or DVD using the Media Creation Tool (run the tool on any PC, and follow the instructions). Put the USB/DVD into your computer, and boot into it. If asked for a key, leave it blank (I don't have a product key). When asked where to install Windows, delete ALL partitions on your drive, and install Windows to Unallocated Space. After about 10-15 minutes you should be good to go. If activation is still missing, do the steps I mentioned above.