QM, i had exactly the same problem a few days back with my airtel... but if u havnt checked it, then do this... especially if u r a verrrryy frequent P2P or torrent user... go to cmd prompt and netstat -n while u are connected to internet. if u see ports with numbers 135, 137, 139, 443... then it seems its a virus... i had port 443 open and ran a complete virus check and got fixed.
after your internet conx seems to get hanged, does the ADSL link on the modem seem to glow?? (if u hav ethernet modem). or do u think the link is cut off? u can ring them to know it.. and if it doesnt seem to hav broken on your side and the backend if they say the link is broken, then change your modem,!!

thats wat worked for me atleast... hope this helps.
/legolas