Magnets are materials that attract certain types of metals, such as iron, nickel, some steel and cobalt. They also impose both attractive and repelling forces on other magnets. Magnets affect the electrically charged particles moving through space and alter the path those particles take from one place to another. Electric charge plays a large role in how magnets work. A magnet can lose its magnetic properties due to heat, fracture or by exposure to a strong magnetic field of opposite polarity. When this happens, a magnet can be rejuvenated back to its previous level of magnetism.
Place the weakened magnet near a much stronger magnet.
Rub the stronger magnet on and around the weaker magnet. This helps realign the poles in the weaker magnet that have been thrown off.
Repeat as necessary to rejuvenate the weaker magnet. If the weaker magnet is already fully charged, there is no way of making the magnet stronger. It is only possible to increase the magnet strength to its initial level of magnetism.
Running an electric current through a magnet could also rejuvenate it, but this is best left to someone experienced to prevent injury.