Is it just Yahoo, or is this happening for any web page?
If it is happening for Yahoo only, check your firewall. Also, it is possible that it is a temporary problem somewhere on the Internet infrastructure.
If it is happening for all websites here are some things you can check:
In both Firefox and IE, click on File and then make sure Work Offline is not checked.
In Firefox, click on Tools>Options>Advanced>Network. Then click on Settings next to Configure how Firefox connects to the Internet. Make sure No Proxy is the only item checked.
In IE, click Tools>Internet Options>Connections, then click on LAN settings and make sure nothing is checked.
If all else fails, click on Start>Run and type CMD and press enter. You will get a black box with some white text in it. type this: tracert yahoo.com and press enter. You will get a cryptic screen that lists all the devices the connection passes through on it's way to Yahoo. The first one you see that starts with 192.168 is your computer. The ones that say myvzw.com are parts of the Verizon network. Then you are likely to see a few others before it ends up at Yahoo.com. This procedure won't fix anything, but it may help to find out where it's broken.