Hi Sam,
Were there any changes made on the computer prior to the issue?
You might get this error if there is any proxy set on the computer or due to a third party program. Follow these methods to resolve the issue:
Method 1:
I would suggest you to follow these steps and check if it helps:
a. Open Internet Explorer.
b. Click on Tools and then click Internet options.
c. Click the Connections tab, and then click LAN settings.
d. Deselect the option Use a proxy server for your LAN.
e. Place a check mark on Automatically detect settings.
f. Click OK twice.
You may also refer to the article for additional information:
Change proxy server settings in Internet Explorer
http://windows.microsoft.com/en-in/windows7/change-proxy-server-settings-in-internet-explorer
Method 2:
Refer to the article to check if Fiddler is the cause of the issue:
[Fiddler] Fix "The proxy server isn't responding" Problem Caused by Fiddler
Let us know if it helps.