Hi there and welcome
To clarify, what exactly happens now when you try to start the Mac?
Power off the mac, then hold SHIFT and power it on with SHIFT held down.
Keep shift held until you get to the sign in screen, then try to sign in and tell me if it then works please.
ok, do the following please
Tell me if you can then sign in
yes please, you need to use all 4 buttons, it looks like you may have missed P
Power off the mac
Then start the Mac and hold down the Command and R keys at startup.
Hold these keys until the Apple logo appears. After your computer finishes starting up, you should see a desktop with an OS X menu bar and an OS X Utilities window
choose Disk Utility from the Utilities window, or from the Installer menu if you're started from an installation disc.
Click the First Aid tab.
Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.
Select your Startup Disk (usually named "Macintosh HD").
Click Repair. Disk Utility checks and repairs the disk.
Let this complete and when it has, tell me if you still have the problem please.
ok, looks like then the hard disk may be failing or corrupted but it is not reporting it
Do you have data on the hard drive you critically need to keep?
ok, in OSX utilities, choose the option to erase disk, then try to install the OSX
Let me know the results please
It can always happen, corrupted data, a bug in software installed, it does not need to be harmful sites