We need a little bit more information than this.
Are you on DSL/CABLE/FIBRE/DAILUP ?
Are you on just a modem, or does the modem have wifi as well?
Are you on just a modem, or is there a router connected to the modem that connects to your mac again?
How does it connect to your mac, are you on Wifi normally, or through an ethernet cable from mac to router (or modem)?
Did your router for example have a per MAC (not apple mac, but hardware mac) address filter and the address changed with your upgrade? or perhaps a static internal IP and the router looks for 192.168.1.10 and you're on 192.168.1.2 now?
When you open the terminal.app type : ifconfig
and it returns stuff, it will have a bunch of stuff, listed on things like en0, en1, etc. Some will say 'inactive', one should say 'active', the address for the active one, can you tell if that's wifi or ethernet? And, is the IP listed behind 'inet' either 127.0.0.1 or 168.something or 192.168.something?
These details can all help narrow down where the problem is and what possible step to take next.
Additionally of course, foolproof check! Go to the system settings and under network check if anything on the left is listed as [green] (little green icon) and see what it says under those details.
If you use any DNS, perhaps change it to the public google dns, this sometimes fixes things as well. Under the 'working (read green)' network, select advanced > dns > and remove what's there, and force it to 8.8.8.8 and 8.8.4.4 and restart your system.
Go into the modem if you can through wifi or (preferred) ethernet and see if you can tell if it has issues connecting to the network or if the modem actually says 'yep, we are activate on the internet', that way you know the issue is with either the isp, your modem, or indeed actually osx. Then go up the chain, if you have a router, check by connecting directly to the router and see if it can get on the internet. Or if it reports back 'i really can't get on' and you know the problem lies there rather than osx.
Before I overwhelm you with even more details and steps to try, pick the first couple and answer some questions, then try a few things and report back. Who knows what you find, and maybe we as a community can narrow it down and try again.
Oh, if you go over wifi, the icon in the white top menu bar for wifi shows bars right? And when you click on it it shows you're connected to your wifi point?