Emily Willis, the famous adult film actress, is still in critical condition following her hospitalization last month -- in fact, we've learned she hasn't regained consciousness.

Michael Willis -- Emily's father -- tells TMZ his daughter is currently in a coma and unresponsive at a hospital, but has stabilized a bit in comparison to the state she was in when first admitted in early February.

While she isn't as bad as she was when she went in, we're told the family is bracing for the worst -- as Emily is now on a ventilator to assist with her breathing.

Michael confirms to us that Emily was in rehab for eight days before she suffered a cardiac arrest while in treatment. While law enforcement sources previously told us that the 25-year-old was brought in for an apparent overdose ... Michael says doctors told the family that her early toxicology screen came back negative. Translation -- nothing was in her system.

FWIW ... Michael says they're all still waiting on a physical copy of the report to confirm this.

Michael -- who is actually her stepfather, but raised her as his own --- recalls Emily eating very little before entering rehab ... telling us that she weighed only 80 pounds when she checked herself in. However, it's unclear if that played a role in her health crisis.

Still, according to Michael, Emily was determined to get healthy and was very open about seeking help for her substance abuse problem.

Emily's loved ones are doing everything they can to help the stricken starlet -- her brother, also named Michael, has launched a GoFundMe to help cover her medical bills and other expenses. Her father tells us they needed to do this as they're unable to access Emily's funds ... which are currently locked and do not list her family as beneficiaries.