How are you trying to make the connection between your PC and your router? Is it via an Ethernet cable or (as would seem more likely from the wording of your post) via wifi?
If you're trying to use wifi, are you sure that your new PC is actually wireless-enabled? (Many desktops aren't).
Assuming that you're using wifi (with a wireless-enabled PC), does your router support a 'WPS' connection? (The vast majority of routers produced within the past decade do so). If you're not sure, please tell us the make and model of your router.
If you know that your router supports WPS (which gets rid of the need for a password), have you tried connecting that way?
https://www.digitalcitizen.life/how-connect-windows-10-devices-wireless-networks-using-wps