Sent a bill that doesn’t exist?!

Hi I hope this is the right place to post this, if not please help point me in the right direction? I had a baby last October, nothing crazy or super out of the ordinary. We paid all our bills by the end of January, it was entirely taken care of. Shortly after that we received a $3300 bill from the hospital for a bunch of things we either already paid for or didn’t exist. We called our insurance who said they basically have no clue what it’s about, they made some calls to the hospital billing people and nobody can find the origin of this bill? They said they would call us back when they got to the bottom of it…So we didn’t do anything because…there’s nothing to do? And they never called us back 🤷🏻‍♀️

Then my husband switched jobs and so we got new insurance. We just received the bill again and were told we’re going to be sent to collections. For a bill that doesn’t exist. Does anyone have any advice? I don’t know where to go from here. I didn’t know what flair to put so I just picked one…thank you in advance!