There should be a little light on the outlet that shows if there is power or not. The FAs can attempt to reset them, but those mid-90's in-flight power systems are all pretty sub-par. My MacBookPro even with battery killed a number of seats on NW.

This is a problem with a number of in-seat power systems that came out in the 90s. The only system that really worked well for me was the DC Empower system on CO. The $30 cable from Apple directly hooked up to the computer and it worked flawlessly for hours and hours.