Chosen Solution

tried safe boot, booting from disk… from backup drives… nothing works. the screen freezes on the apple logo… is the motherboard fried?

This sounds like you have a bad hard drive, or a bad boot environment. Try this link to reset your PRAM and SMC here: Resetting PRAM and SMC. I would also try to boot your machine off of a CD if you can.

just reinstall the mac os using external usb having os bootable or

  1. turn off ur mac
  2. power on while holding command and r keys simultaneously, this will boot ur mac in recovery mode
  3. follow steps on screen and ensure are connected to the internet if that doesn’t work replace HDD or connecting cables as d case may demand

Ha..ha..ha..I know this is not funny at all. I have this mac pro early 2008 for more than 6 years and it is still working hard till today. Did you guys check the two small buttom battries that sit quietly on mother/ logic board, usually unseen behind the graphic card? Mostly, this is the main culprit! I had some experiences with the s/s above that being resolved after replacing them with the new one. To be safe, replace them every 2-3 years and check its surface contact for any sign of humidity or oxidation. The 2nd possibility is new inserted RAM or maybe the RAM is death. But this rarely cause the symtoms you describe above. Just check on system information about RAM. Then, this one is not easily solved or tracked. You may have accidentally download/ run incompatible software (software conflict) or deleted the important one. Just think what you have done recently with your Mac and search the culprit one by one.