How do I re magnetize a magnet that has lost its strength. 
You ought to be able to re-magnetize a magnet the same way you magnetize previously non-magnetic metal – you can stroke it repeatedly with another magnet, always with the same pole and in the same direction in order to line up a preponderance of magnetic domains in one direction (do it the same way the magnet still has any residual magnetism, so as to avoid wasted effort). Or you could orient it to the Earth’s magnetic field and astroike it sharply with a hammer. Or run coils of wire around it and apply current so as to make it an electro-magnet, and some of that effect ought to stick.
How did it lose its magnetism? A sharp blow can do this, as can raising the temperature to above the Curie Point.
It is a magnetic clasp on my pocetbook. The clasp was left open and was not in the closed position for a long time. That’s how it lost it’s magnetism. Cal do you mean a sharp blow like with a hammer? I don’t know which area of this clasp is north or south pole. Thank you Cal
If it’s a ceramic magnet (and if it’s part of a purse clasp, it very likely is) do NOT hit it with a hammer–it will shatter. You can try rubbing it vigorously with another strong magnet, but ceramic materials have a relatively high coercivity (resistance to being magetized, for the layman) so it probably won’t work all that well. In the factory where such magnets are made, they use a magnetizing coil–a few turns of very heavy wire through which a large DC current is passed. For a brief moment inside the coil, an extremely powerful magnetic field is produced which is enough to overcome the coercivity and magnetize the material.
I don’t think I’d try to re-magnetize such a thing. It’s probably easier to repl;ace it. Sorry (I come from a tradition of metal magnets. It still bugs me that refrigerator magnets are flexible.)