A demurrer essentially advises the court to examine the complaint to see if it states a cause of action and if it does not, then the complaint needs to be dismissed or amended, depending on the facts alleged and circumstances in the complaint and demurrer. The facts are assumed true, for purposes of the demurrer, which only challenges whether legally sufficient facts were alleged in the complaint. Sometimes, the parties can offer documents, provided that such documents are proper for judicial notice, such as government or court records.
if the complaint's allegations are sufficient, then the court will deny the demurrer. If the complaint's allegations are insufficient, the court may give you leave to amend, if you can correct the defects or deny you leave to amend if the court determines that you cannot correct the defects. At this point, you have a choice of amending the complaint, opposing the demurrer, or doing nothing and taking a chance that the court will review your complaint and agree it states a claim. Any opposition must be filed with the court at least nine court days before the hearing date.
You did not provide enough facts to give any specific response, but you appear to be complaining about a medical procedure that went badly. If your complaint is for medical malpractice, then there are very specific things that must be presented with your complaint. Professional malpractice by a "health care provider" has many requirements. See, for example, Cal. Code of Civil Procedure, Secs. 340.5, 364. Perhaps you can review the demurrer to determine the nature of the defendant's concerns and correct it, if possible.
I would urge you to use Avvo's "find a lawyer" tab and contact a lawyer with medical malpractice as one of their areas of practice.
Consider whether or not the defense has a point about your complaint. Did you fail to plead all necessary facts or otherwise produce an objectionable complaint? The demurrer will tell you what the defense thinks the problem is. If you agree that your complaint is lacking, contact defense counsel and see if they will stipulate to you amending your complaint to overcome the objections.
On the other hand, if you think your complaint is good and not objectionable, oppose the demurrer (in writing and pursuant to the CCP) and tell the court why your complaint should stand. In any event, ask the court for leave to amend if the demurrer is to be sustained. That way you can try again and wont lose your cause of action if the demurrer is successful.
Finally, you can always consider consulting an attorney to help you.
Best of luck to you.
This response is intended to be a general statement of law, should not be relied upon as legal advice and does not create an attorney/client relationship.