It is imperative that you get immediate intervention for your grandaughter! It is not a case of whether or not she is serious, it is an issue that requires emergency intervention. My suggestion would be first that if she is willing to go to the hospital, GET HER THERE ASAP! If she wont go, call the police and have her taken against her will, I know this sounds extreme but trust me when I say that someone with suicidal ideation is not someone who can make a decision in their best interest!!!
I am not sure what has happened since you have posted but before you do anything else, you must get help NOW!
I hope that this gets to you as soon as it can.
There are also crisis intervention numbers that you can access, the 911 can give the number for prevention.
I will keep you in my prayers