Try resetting the TCP/IP stack. (It's fixed loads of problems for me).
If you're using Windows 10, follow the instructions under 'Solution 1' here:
http://windowsreport.com/wi-fi-valid-ip-configuration-windows-10/
For earlier versions of Windows, scroll down to 'Let me fix it myself', and then to the appropriate version of Windows, here:
https://support.microsoft.com/en-gb/kb/299357
If that doesn't work, return to my first link for some further possible solutions.