Forgotten password on login page

Try looking on the modem itself, sometimes they have their default username and password written on them. Try combinations of "user" or "admin". eg username: "admin" password "admin"
if these don't work and you don't know why you can't log into your modem, try resetting your modem using the reset button on it (or holding the power button for 10-20sec) and then try the above suggestion again.
If you still can't login, write back with your modem make and model number.