Is your boot drive selected in the Startup Disk pref pane? Double-check there, and you will need to unlock that pane to select the boot drive.
In your previous posts - you had a 2012 MBPro, and running Mountain Lion.
The boot up process is considerably different between Mountain Lion, and El Capitan. Most Macs changed the boot screen from grey Apple on a light screen (Mountain Lion to Mavericks, to the newer white Apple on a black screen (Yosemite and newer). The progress bar simply shows you that the boot is proceeding.
If you still have a spinning hard drive, then you will find out that El Capitan runs MUCH better on an SSD.
If, instead of going through the Apple icon boot process, you see a light grey screen (showing a blurry background for a few seconds), and a multi-step progress bar that is white, then that is simply waking up from hibernation - a deeper level than the normal sleep mode.