had this authentication failed problem few days back..when called next day i was asked to follow these steps:

>restart your router
> check internet light, if its on or blinking than u need to call customer care but mine was off
> in internet explorer, address bar 192.168.1.254 you will be directed to thomson page
>there you will see internet status disconnect
>on the left side, second option is thomson gateway>go there
>click set up in the pick up options
>then u will b asked user name n password, that will be you landline number and password 123
>and wait till set up completes
your internet should work by now if not thn call customer care...thou the representative accepted that there was some problems in Qtel server previous day and hence the settings were to be reset.