I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
The statute of limitations for breach of contract in California is four years, so if you received services in 2014 that weren't paid for, even if it was due to a mistake in address, it is still necessary to pay that bill. The hospital is owed that money, whether the correct address was given at the time of service or they made a mistake mailing the bill or not. Not receiving the bill is unfortunately not a defense to payment. If they wind up filing a lawsuit and your defense is that the bill was mailed to the wrong address, the judge will rule in their favor. Reprinting the bill with the correct address unfortunately doesn't void the original debt.
Under the Fair Debt Collection Practices Act, a consumer has a right to send a creditor a written request for validation of a debt, via certified mail, and the creditor must cease all collection efforts until that validation is sent. They have 30 days to reply. You're also allowed as a consumer to request that they cease all communication with you (also sent via certified mail). You should be aware that sending that type of letter could induce them to file a lawsuit, but they cannot sue if you've requested validation they haven't given you yet. Violation of the FDCPA carries a fine of up to $1,000, so if they won't send validation and they wind up suing, you may be able to file a countersuit.
Also note that, if they sue, you can raise any defenses you would have had against the hospital had they sued instead of sending the debt to collections.
Here is a sample letter that might inspire them to send you the documentation you're asking for:
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