Insurance company charged me for 30k and Seeking Advice

[Elsa Hsu]

My property is in an upper unit in a condo with an HOA. (picture below)

Issue: One day, a pipe under the floor leaked from your unit into the downstairs unit below through the kitchen AC vent, causing damage to the neighbor's kitchen. (The pipe is between the ceilings of the upstairs and downstairs units.)

My condo was paid off, so you didn't have house insurance at that time. You bought house insurance on the same date but too late. That means I don't have insurance company to fight for. 

The letter from the neighbor's insurance company:
National Subrogation Services, as recovery agent for Mxxx Insurance Company has investigated the facts and determined the accident was due to your insured’s negligence.
Due to the payment to Mr. XXX, we are pursuing the subrogation rights of Mercury Insurance company. The amount we are claiming is the amount paid plus the deductible as noted above. We have enclosed documentation to substantiate our claim.

Actions that I have taken so far:

1. I asked the plumber to prove I am not negligent (still waiting for the statement; it seems like the plumber does not want to do it).

2. I asked my HOA to provide the Condominium Plan for the insurance company. The HOA answered me, saying, "They don't know what that is!"

First of all, this has taught me how important house insurance is.
Can the BP family guide me on what I should do to prevent this Big Charge, or can you recommend a Insurance Policy attorney in California for me, please?  
My area is in LA county. 

Thank you !!!!