So to summarize -
Is the network physically connected? Typically both the computer and router (or switch) have a link light; often link and traffic lights. Unless you have a link light at both ends your cable is broken somewhere in between.
(I have seen cabling which links only in one direction…)
Do you get an IP address? Unless this is a brand new setup, or some complex setup with server, your router usually hands out IP addresses and if it has stopped, something is up with the router.
IPCONFIG command will tell you if you have an IP address and what the default gateway is. If it has stopped handing out addresses, you need to configure the router. Logging into a router and setting the configuration is a mid-level task. Trying it as a newbie could mean losing the configuration to the internet.
(Hopefully your internet connection is like your home PC - DHCP, which means it gets the external address automatically. If it’s PPPoE (phone company) it may need a login… If it’s 192.168.100.something then odds are your router is playing second fiddle to the network modem box which also acts like a router…
Do you run a VPN? That will hijack you IP settings.
I have seen end user VPNs that make your PC forget your router (“default gateway”) and never get it back when the VPN is turned off. In this case, it helps to know what the router address and a good internal address for the PC could be. Then hard-code an address into the PC.
(I.e. router is 192.168.0.1 - so use that as default gateway, odds are 255.255.255.0 is the subnet mask, and pick a random unused number like 192.168.0.77 as the PC. )
Also watch that for some reason the WIndows firewall is not blocking traffic.
Short answer - if it’s not wiring and not fixed by a reboot, seek a nerdy friend for help…