When attempting to start my computer version 5.1 Mac Pro, it will not start up. If I try to do a TARGET disc, I get a fire wire icon, if I try any of the other diagnostic keystrokes to start, it goes about halfway on the progress bar and then goes black then the no signal text appears.
Running iOS 10 Mojave.
Mac Pro, macOS 10.14
It sounds like Target Disk Mode is working as it should, hence the Firewire icon.
Once boot into TDM, then you may connect the Mac to another and access the startup drive that way, and potentially run Disk Utility's First Aid routines on the drive and its volume(s).
Please see: Transfer files between two Mac computers using target disk mode - Apple Support
It sounds like you may have a failing/failed startup drive.
Have you tried to restart in Recovery mode (Command-R)?
If you can coax the Mac to start in any Recovery mode you might be able to use Disk Utility to run the First Aid routine on the startup drive and volume(s).
Use macOS Recovery on an Intel-based Mac - Apple Support
Is this your only Mac?
I assume Safe Boot with SHIFT key doesn't work?
Does holding Option key on startup show any boot choices?
Boot failure
