Can you please tell which firmware you are using??

As per my knowledge, in Stock,Toamto and DDWRT firmwares of Netgear WNR3500L, the http username and password are written on the nvram section.
You can perform the following steps ...
* Get the access of router console either using ssh or telnet.
* nvram get http_username
* nvram get http_password
Use these info while login through WebUI.

If the above nvram operations show encrypted output then you can manually set those parameters too ... using the commands below...

nvram set http_username=usrename_you_want_to_use
nvram set http_password=password_you_want_to_use
nvram commit