Feel along the length of the power cord, bending and flexing as you go, to check for any kinks or breaks. Check the ends for any broken connections, such as plugs pulling loose or spots that may have gotten chewed by a pet or caught in a vacuum cleaner. Inspect the AC brick. Is it discolored? Are any parts warped or expanded? Give it a sniff—if it smells like burnt plastic, that's likely where the trouble lies.