Mac cannot start up Yosemite

Question

hi
i have a Mac with Yosemite, is taking more than an hour to start up, the bar before request login takes an hour and the login window does not show up
i have Yosemite
help !

Answer 1

You could try & see if the computer may eventually start into
Safe Mode or Safe Boot, where you'd hold the Shift key down
for a long time just after startup, and until there is evidence of
the Finder window appearing. Sometimes this can take up to
15 to 10 minutes, depending on several factors.
•OS X: What is Safe Boot, Safe Mode? - Apple Support
Once into the system, other tests and perhaps Console error
logs from the System could be viewed. You could also try &
see if Disk Utility could repair permissions or repair disk, once
you have the computer running. You may be able to start up
into OS X Utilities on the Recovery mode, to check/repair HDD.
If you get as far as a Gray Screen, and no further, this may help:
•Mac OS X: Gray screen appears during startup - Apple Support
•OS X: About OS X Recovery - Apple Support
More complex & advanced methods of attempting to diagnose & repair
issues on startup can be rather involved and some of the methods do
require a high or greater degree of understanding the Terminal or the
command-line access to the underpinnings of OS X. This goes behind
the scenes where the user interface is not simple or user-friendly...
For those who have read & carefully tried the command line access
and not cause more damage, once learned it can be helpful & direct.
•Mac OS X: How to start up in single-user or verbose mode - Apple Support
Use of command line and Terminal access is not recommended in
general by those who have a problem and only one computer to
learn how to use it; you may have a worse situation due to lack
of experience. So other methods less invasive (disk utility) are the
recommended path; or if problems point to hardware cause, then
the computer would best be taken to an Apple Specialist or genius.
•Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support
So if you can access the computer to get the basic report mentioned
in the above post (etrecheck) that may show someone some clues as
to what may be wrong. However a command line report such as that
which uses terminal commands, is good if read through by someone
who understands what the cryptic details mean.
Hopefully you can post more information because there are several
things that may contribute to your computer's symptoms...
To troubleshoot involves attempting several things to see what works
when working in the blind on resolving problems by rote. However if
you haven't experience in the Terminal or command-line, tread careful.
Good luck & happy computing!
edited