I'm curious as to what you're trying to do or access. as the HP Laserjet P2015DN does not have a default web password, or a web password at all unless a network administrator has set one on the network, in which case you probably will not be able to reset it by doing a cold reset. To make sure you're performing the cold reset correctly:
1. Turn off the printer.
2. Press and hold the Go button.
3. Turn the printer on, and continue to hold the Go button for at least 5 seconds, but not longer
than 15 seconds (go for 10 seconds to make sure you're past the 5 second minimum). During this process, the Attention LED turns on.
4. Release the Go button.
The following only works if your printer is connected to a network using the built-in network interface.
To open the embedded Web server, type the IP address or hostname of the printer in the address field of a supported Web browser on a computer on the network. To find the IP address or hostname of the printer, print a Configuration page at the printer by pressing and holding the Go button (with the printer on) until the green light blinks, then release.
If you're trying to set the serial number, then you need to send a command instruction set thru the USB cable connected to a local computer, this is where you'd use the name HPBOISEID in the text to access the service mode function to set the available options.
Best of luck.
Edit: There is no password. I have the service manual, I work on these printers & have never needed any password, nor can I find any reference for one. T-bar needs to heed his own advice and investigate his own answer. His link is for a printer supplies website, not a printer support website, and the password request link given is for your account password for the website, not any printers.