It looks like it's not in the guidelines or FAQs anymore (perhaps because it's generally-enough understood that HN is largely independent of YC), but it's outside the purpose of HN to try to contact or raise personal complaints about YC companies.
Can't help you on the YC front, but have you tried doing an unclaimed-property search in California (their HQ) or whichever state you were in when you made this investment?
You should file a complaint with the CFTC
cftc.org
I think you can also do a lookup on their site to see if there are any open cases and investigations.
Well, investing in private companies for equity, requires taking a risk. Looks like your risk was very high and company went bust. For all intents and purposes, you have lost your investment. If there is anything left over from the dissolution of this business, it must b being held by a board of trustees. You need to make your research and figure out if such residual money exists somewhere and if it does, go after it, preferably with a lawyer;s help, if your investment amount justifies it.
There are contact details for YC on the YC homepage: https://www.ycombinator.com/contact/