can not reconnect to internet

Hi,
My computer cannot re-connect to the internet after it
has disconnected. I have a 56k dial up modem, when I
first connect it works ok and I can view web pages and
download e-mails fine. If I manually disconnect I can re-
connect ok, but if the connection is lost for other
reasons then it will not re-connect. The modem dials and
the dialogue box says 'verifying user name and pass word'
but the connection never completes. If i restart the the
computer it will connect ok again until the connection is
lost.
Help, thanks