Yikes, that’s always a pain. I’d start by removing as much of the pooled water on top as you can with a submersible pump or a heavy-duty wet/dry vacuum so you don’t dump all that gunk into the pool at once. Then scoop or siphon any big debris from the pool itself. Once the water level is normal, give the pool a good shock and run the filter nonstop for at least a day or two, backwash or clean the filter frequently because it’ll clog fast with all the extra muck. Brushing the walls and vacuuming the floor after the shock will help get the algae and dirt under control before it sets in.