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Ask HN: Founder claims I never purchased options, but I deposited checks

My previous company (Bay Area startup) was acquired by a Fortune 500. I reached out the founders, congratulating them and asking about my options. The Fortune 500 lawyer team responds saying "[they] do not have a record of any shares held by [me]". I reply back with a copy of a check I made out to Bay Area startup with a Memo explaining that this money is to purchase the options. The check was cashed and endorsed with the Founder's signature on the back.

Other than this check and the 83b election document, I don't have any other documentation about how many options I own.

PLEASE HELP. I don't know how to proceed. Do I begin Arbitration or find an attorney?

I am totally put off right now, feeling betrayed by the founders.

Edit: Thanks everyone for the warm thoughts and advice--I'm looking for a lawyer now. Really glad I posted.