can i hold the health insurance responsible if something happens since they won't cover a prescription?

There are actually two different answers to this question depending upon how the insurance was obtained. If she is covered under a policy obtained through an employer sponsored plan (which is the case most of the time), then the issue falls under a federal statute called ERISA. Under that scenario, all litigation MUST be in federal court, and insurance company decisions are divided into two groups - "coverage" and "therapeutic."
If a decision is determined to be a coverage decision (e.g. they will cover drug x but not drug y), and there is no specific reason why drug y must be used instead of drug x, then the only thing for which the insurance company can be held liable is the cost of the drug they denied, regardless of what happens to your daughter. I know this is harsh, but the issue has gone all the way to the US Supreme Court, and they ruled against the patient (reluctantly), saying that they could only rule on the law as it currently stands, and it is up to Congress to change or amend it.
If the decision not to cover a drug is therapeutic in nature (e.g. they will cover one dose of a covered drug, but not the one the physician things is needed), they they may very well be held liable for any adverse outcome of their decision. There is, however, a very fine line as to which type of decision it is in most cases, and only a knowledgeable Health Care Attorney (one who understands the implications and case law surrounding ERISA's application to health care) will be able to determine whether you have a legitimate case against them.
The second possibility is where the health insurance was obtained from other than an ERISA covered plan. In that case, any action can be brought under state law and/or using "Common Law" causes of action such as "negligence," and the outcome could be substantially different.
Finally, it also makes a difference what stage things are at. Has the decision been appealed? Have you gone through the administrative process yet? All of these are important before any real answer can be given, but any action you take should be undertaken as soon as possible both for your daughter's sake, and from a legal perspective. I had a patient in exactly this situation when I was practicing Medicine. After two weeks of trying to get my patient covered, without success, I wrote the company a three paragraph letter on my legal stationary and faxed it to them. She was covered within 4 hours! It's a sad commentary on our health care insurance system (and a good reason for a "public option"), but true.