Q: how to restart a MacBook Pro

how you arrived from password to virus is unknown, but at this time there are no mac virus reported by any securities lab in the world so scratch that off the list

if the system is booting and asking for a password this is the default setting of OS X, you need to provide a password for the account or you must know the master password to get into the system.

This is a security feature, not a virus.

OS X El Capitan: About the master password

secondly you may have a firmware password enabled, in which case see this document.

Use a firmware password on your Mac - Apple Support

To restart a Mac please read OS X El Capitan: Log out, sleep, restart, and shut down.

More information would be helpful. Describe the circumstances in which you are being asked for a password.

Solicitation for your password or similar requests for credentials should never be a surprise. Do not provide your Mac's password or any other personal information just because some random popup demands it.

There are no OS X viruses so forget that. If a popup window appears alleging the existence of "viruses" on your Mac, it should be considered a criminal attempt to defraud you. Please read Phony "tech support" / "ransomware" popups and web pages.