Janis,
Hope your neighbor is OK.
1. Bring your water to shock level. Per the Best Guess Table, for your CYA level of 40 (assuming you meant CYA where you typed CA), that's 15 ppm which will take about 2.5 gallons of 6% bleach in your 20,000 gallons of water.
2. Test the water frequently (3x per day, preferably) and add enough bleach after each test to bring the free chlorine level back to 15 ppm.
3. Keep this up until the water is crystal clear and it holds its free chlorine level overnight.
4. It may take several days. During this time run your filter pump 24x7 and vacuum debris and dead algae to waste if you can do that with your filter. Backwash (or clean if you have a cartridge filter) as needed.
Good luck.
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