Extension cords are usually items that get lots of hard usage and are very susceptible to damage. This is one reason that OSHA requires a visual inspection of all flexible cords (including those on power tools) each day or before each use.
A proper cord inspection includes an examination of the jacket for the entire length of the cord. Look for cuts in the jacket that expose the colors of the inner conductors, Once the integrity of the outer jacket is compromised, the cord cannot be repaired with tape or used in this condition. The cord may be cut and UL approved male and female plugs affixed to the ends. Look for damaged strain relief, loose or missing prongs or ground pin, evidence of heat, such as smoke or melted insulation, pinching or discoloration.
Remove damaged cords from service until repaired or disposed of by a qualified person.
Never forget to use GFCI protection when using an extension cord. It can save your life!