Macbook Pro can only startup with external hard drive connected
If disk utility does not show your internal disk anymore it is a Hardware, not a Software fault. Unless you are savvy enough to troubleshoot the hardware issue yourself you will have to take it to a repair center. 

Most likely it is actually not the Hard Drive but the Hard Drive cable that is failing. It is a common issue with your model and you might be eligible for a free replacement (cable). Be sure to either contact Apple Care or go to an Authorized Repair Center and have them check the issue for you.