what model Mac pro, how much RAM, what graphics card, are any red light on next to DIMMs after such a failure ????
what is showing on the screen?
The solid Apple is not in the Mac's ROM at startup. The Apple logo can only appear when it is fetched in the first "blob" of software loaded from a known place on the boot drive. Then a whole lot of stuff is initialized.
The next step requires a lot of files by name, so the File System is initialized, and the Boot Drive is Mounted. If the drive directory is damaged, the drive can not be Mounted, so your Mac begins one pass of Disk Utility Repair. This will take an additional about five minutes.
at the end of that process, it will attempt to Mount the drive again:
-- if the drive Mounts, boot-up continues.
-- if the drive cannot be Mounted, your mac can do nothing more, so it powers off.