DON'T DRAIN IT !!
Just thought I'd throw out the warning until someone in the chemistry area kicks in with more specific info. No need to drain, just fill while you can. Draining will either cause liner problems like wrinkles at the least, or destroy it at the worst. In the meantime get the leaves and any other debris out with a net. Posting the type of equipment, especially the filter type, and any test kit info you may have will be helpful. If you don't have a kit or are not sure how to use it, take a water sample to a pool store and have them run the numbers and post here then you can get detailed advice. Run the pump and filter for a few hours before taking the sample. Whatever you do DON'T LET THEM SELL YOU ANYTHING. If you haven't already, browse through the sister site http://www.poolsolutions.com and do some reading. Lot of info there.
Al