The first thing to check is that the connection itself is OK. Take your PC and give it the IP address of the firewall, or whatever you have hanging on the ISP connection, connect it directly to the service provider router, and see if you can access the Internet. Don’t forget to set a DNS server address, otherwise you can not access webpages by name. Or if you own the final router, do the test from there. I use the IP address of the DNS server at my ISP, it responds to a PING and is a fairly easy IP address to remember.
If that works,then your problem is with your equipment. Check what changes have been made, and undo them !
Check each piece of equipment, working from the Internet connection backwards into your network, and use the PING test on each one to check if there is Internet access. This presumes that you don’t have a firewall blocking outgoing PING (obviously).
Once you find the access stops, then you can focus on which piece of equipment is causing the problems.
If the first test fails, then call your service provider, and get them to fix the problem with their network.
The most likely is the service provider connection is faulty, but sometimes equipment fails, so you need to do the checks each time to be sure.