Did it get hard because it cooled, or hard because it got over-cooked (burned) or seized (if it came in contact with water)? If it's because it cooled, just warm again for a few seconds on 50% power to re-melt. If you over-cooked it, you can try to salvage it with a bit of shortening (Crisco), which might bring it back to the right consistency. If not, you'll have to toss it.
The chocolate that you use makes a difference - Candy Melts are the easiest to work with and have a stabilizer in it that makes melting smooth and easy. The "bark" stuff sold at Wal-mart is also supposed to be easy to work with. Chocolate chips can be touch and go, depending on the brand. Some chocolate chips, particularly white, have a wax that prevents them from melting easily.
For your next batch, if you're using a whole bag of chips or melts, microwave for 1 minute at 50% power, then stir. Then continue to microwave for 15 second at a time, at 50% power, stirring each time until they are smooth. Make sure your bowl and spoon are dry - chocolate and water don't mix.