Your battery went from full to dead because they were discharged by use I suspect. What you use and how much you use "them" will deplete the batteries. If all you have is solar and it has been cloudy I highly doubt they are able to generate enough power to charge the batteries. Solar panels don't just "quit" when cloudy but will only produce a very small amount of electrical power compared to a bright sunny day (maybe 10-15%). Also, depending on the size of the solar panel(s) the amount it produced may have been barely adequate on a sunny day much less on a cloudy day.
Here is a thread that may be interest to you pertaining to a similar problem;
http://www.keystoneforums.com/forums...t=dead+battery