Hi, your problem could be due to any number of things and a little patience will be needed to try and pinpoint a fix. Here are a few things that could help, and are worth trying.
If only 'Server not found' usually indicates no interet connectivity. However if you have connectivity through other programs then try a reboot. Close out to home screen first, then double tap home button - press on your browser icon till it wiggles, then tap the '-' red icon to close it. Hold start and home buttons in together until apple logo appears to reboot. When back at home screen check in settings that you have either wifi or carrier connectivity.
If only affacting Safari, either a restore or, checking (wifi) that the relevant port is open in your router (check with safari on your PC). If all is well then try a restore.
If a third party App, try closing the App first as given above - reboot and try again.
If the server is unreachable for another reason - say it is an apache server then you will usually have this given as a reason with the server not found message - this is normal and due to restrictions placed on the 'forest' that your internet domain has access to.
http://support.apple.com/kb/TS1629 gives the well known ports used by Apple applications and services.
If this is a recent problem that you have only just encountered, check all is well with your PC's connectivity, then proceed as for Close App, Reboot. Also try tapping the address to select it then tapping the enter key to resend.
It is possible that certain addresses at any given time are not accessible due to routing restrictions for a variety of reasons. Accessability by one browser is no guarantee that another will use the same routing to arrive at the same address. Occupational hazard of using the internet. if the same addresses are still problematic say 24 hours later then try contacting your data or ISP provider to see if there is a problem at their end - it can happen.
The following link will also provide some things you can try if you wish
Best Regards, Ct