• Try safe mode if your Mac doesn't finish starting up - Apple Support
This says Yosemite, but information will work on most other OS X:
• OS X Yosemite: If you see a blue screen at startup
There may be a hardware problem with the computer, however that
may be difficult to troubleshoot if you cannot boot from OS X install
system install DVD, to see what it's version of Disk Utility says about
the main hard disk drive. The included Hardware Tests are not a sign
of confidence nor are their results conclusive. (Basically unhelpful.)
• ID Your Mac - iFixit info: (non-portable desktop models)
https://www.ifixit.com/Info/ID-your-Mac/826
Would be helpful if you have any information on the build model Mac
and if the unit is a Tower, is aluminum or plastic, what it looks like, etc.
In any event...
Good luck in this matter! 🙂