1st, try AGL's internet phone tech line support,
If you want to see something tangible immediately then try the below.
Failing that then have a look see here.
If everything looks good there then try the tips here.
If still no luck then I would recommend ringing AGL direct again to see what can be done, This depends on a few factors IE: NBN issue or Southern Phone issue.
My post takes into consideration that the modem settings have not been dropped as I have commonly seen in the past.