In my experience, this is indicative of either a failed hard drive and/or a failed hard drive I/R cable. If @xcesv4c Matts solution allows you to boot but you still have issues, replace the cable first. Use one from the 2012 13" machine even if some mods need to be made. The 2012 is a more substantial cable. If the cable fails it will cause issues with the hard drive and sometimes kill it. After the cable is replace, reformat it and select the "write zeros" option to map out any bad blocks. If the drive still fails, replace with a Seagate SSHD for a speed increase.