Recently, I faced a strange error suddenly in my gmail inbox when, I saw gmail chat was disabled some how and I could not chat with my gmail buddies from the gmail interface, but I was able to chat with them using desktop based gtalk program.
Chat window shows the error, ‘Your Network administrator has disabled chat in Gmail,’ your network administrator or ISP has chosen to block chat. If you’ve verified that your network has not blocked chat, as a first step, please ensure you have a good Internet connection and refresh your browser
After some googling I came to know that this can happen if some how browser cookies, conflict with software on your computer, browser cache that needs to be cleared, or a temporary problem on the Gmail server.
This can also happen when some Internet security, firewall and anti-virus applications can prevent Gmail from working correctly, so just these software if they are installed on your computer, you check if they are blocking google gmail browser chat.
In my case, I just cleared the browser cache in google chrome and the issue was fixed, I deleted the browser cookies and history in google chrome as shown in the image below.
Note: If you are using google chrome then you can press Ctrl+Shift+Delete to delete all this history.
The procedure for clearing the browser cookies and cache could be different for different browsers, so dig out the option, if still you are not able to do let us know in comments we would love to help you in that case.