2009 iMac displaying "ERR_CONNECTION_CLOSED"

Hi Mac community,

I logged in this morning to discover that my 2009 iMac is no longer allowing me to access the internet. I have full connection to the usual WiFi network and Ethernet is working fine, but upon trying to load a page I get the error "ERR_CONNECTION_CLOSED". It won't allow me to connect to any websites. I have reset the DNS, the Proxy and flushed and closed the socket pools multiple times. Alongside this, my Spotify and iTunes say that there's no internet connection and Safari won't connect either. My Firewall is usually always off and I've run network diagnostics to no avail. Connecting a windows laptop to the computer allows the laptop to connect to the same wifi, and it works fine on the other device, so the problem is central to the computer, not Chrome.

The Mac was working fine last night. In the past couple of days I updated to Sierra 10.12.6, so that might have something to do with the issue. Note also that I sometimes have a socket not connected or closed connection problem when the WiFi has been turned off and then back on again for a few hours - this requires a restart to fix. Multiple restarts are not helping currently.

Any help would be much appreciated as this computer is my main one - thank you!

Mac Details:

iMac (21.5-inch, Late 2009)

3.06 GHz Intel Core 2 Duo

4 GB 1067 MHz DDR3

ATI Radeon HD 4670 256 MB