That's too bad.
I always consider situations like this to be a blessing in disguise. Mama platy gave her life to give you a warning, hopefully in time to save the rest of the fish.
First of all, take a look at the following article. (if you already know about the nitrogen cycle, ignore it

)
http://www.fishlore.com/NitrogenCycle.htm
This is the single most important stage in your tank's development.
You've learned now that you've got to vacuum the gravel.
Get a test kit, get the tank's ammonia, nitrite, and nitrate levels under control, and you'll have a healthy setup for your little platys.
Good luck and post any questions you might have.
