#1 Turn off and back on the power to the modem combo.
#2 Then try to access it, using it's LAN IP Address.
#3 If that fails, go get the manaul / user's guide for the router.
#4 You will need to know:
a) the default user name
b) the default password.
c) how to setup this modem combo so that you can access the net.
d) how to reset the modem combo back to the defaults.
e) how to change the default password.
#5 Once you have that info, reset the modem back to the defaults.
#6 Re-setup the modem so that you can access the net.
#7 Be sure to change the default password to a strong password.