That's not the usual Windows password you use to login into Windows. That's the BIOS password and that could very well be different.
Now look in the user manual or ask Dell tech support how to reset it in your model. If it used to work normally, and now suddenly pops up while you did nothing that could have caused it, it could be a hardware issue you can't solve yourself.
I turn on my Dell Optiplex it goes to a black screen that says enter system password. When I try to type it in, nothing happens like the keyboard has been disconnected .. it makes the sounds but nothing types in. I can get out of the screen no matter what I do.