Why Is It Saying Server Not Found?

When clicking on Firefox, it says server not found, if I click on IE it says not connected to network. We just replaced the AT&T modem because last one fried. If I manually turn off computer at tower and reboot it will usually work but not for long. The wifi works fine with our hand held devices so I know AT&T won't help. No issues with previous modem, I didn't change anything with the connections or the computer itself. Any ideas on how to resolve or troubleshoot this? Running windows 7 on a hp

 

The behaviour suggests that the tower wifi adapter driver needs updating. You could try to do this through the computer manufacturer's support pages, or through Device Manager, or by running a free driver updater such as Iobit Driver Booster or one of the other driver updaters listed HERE. If you use Iobit be careful to uncheck any boxes that download or install Advanced System Care or other Iobit software - unless you want it of course Whichever driver updater you may use, only allow it to update your wireless adapter.