Hi Digger!
I had the same problem a while ago. It could be due to many things, but try the following. I call it the 1,2,3,4,5 method.
1. Uninstall the game.
2. Clear your cookies and cache data. (I use a program called CCleaner, there is a free version available which is more than capable, just google it)
3. Re-boot your router. Switch it off or unplug it, leave it for a few minutes then turn it back on.
4. Now you have CCleaner, look in the menu on the left hand side for the 'registry' option. Click 'analyse', then 'fix all'. You'll be surprised at the amount of junk it will find.
5. Your system should now be free of any junk left over from previous installs of Dogfight and any other programs that may have been causing you problems. It is now safe to re-install the game.
This fix should get you back in the skies. It tends to work for 99% of problems on Windows 8.1.
CCleaner also has a 'start-up' option which allows you to monitor what programs automatically open on start-up and allows you to enable or disable them. This can be a very useful tool too!
I hope this gets you back in the skies, my friend!