Prior to Verizon in home agent you could use the user name admin and password "admin" or "password" to directly access your fios router. This changed with the introduction of "In Home Agent" program. You now must create a new password using the "In Home Agent" program and then use that password to access your router. The user name will remain admin.
You do not have to use in home agent. Many of us refuse to even install it.
After a reset your password is either
password that is printed on the label
not set and you are prompted to set it
Some say it can also be the serial number of the router. I have never seen this on a reset although it is what they changed it to for those routers that used to have "password' or "password1" after they were initially installed