MacBook Pro won’t load to login screen

Sounds to me like the disk has been damaged. Depending on what damage was done you may lose all your data in the process of fixing it (assuming no actual hardware failure.)

Repair HD and Permissions - Lion through Yosemite

1. Restart the computer and after the chime hold down the Command and R keys until the Utility Menu appears.
2. When the Utility Menu appears select Disk Utility then click on the Continue button.
3. After Disk Utility loads select the indented (usually, Macintosh HD) entry from the side list.
4. Click on the First Aid tab, then click on the Repair Disk button.
5. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported.
6. If no errors are reported click on the Repair Permissions button.
7. Wait for the process to finish, then quit Disk Utility and return to the Utility Menu.
8. Select Restart from the Apple menu.

I'm afraid that not knowing your essential machine info I'm unable to know what may be needed. Try the above to see if it works. If not please relate exactly what happens and report any error dialogs. If you feel uncomfortable doing the above then you should make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple. They can fix it up for you.

OK. I'm afraid we are just banging our heads on the wall. The following may be helpful:

How To Do A Factory Reset

Selection A should be used on computers that came with Lion or later when factory new. Selection B is for Macs that came originally with Snow Leopard or earlier.

A. Factory reset your Mac - Apple Support

B. Factory Reset Your Pre-Lion Mac

Follow these instructions until you get to Step 5: Factory reset your Mac - Apple Support. At Step 5 you will need a Snow Leopard DVD or the installer disc that came with the computer.

1. Boot the computer using the Snow Leopard Installer Disc or the Disc 1 that came with your computer. Insert the disc into the optical drive and restart the computer. After the chime press and hold down the "C" key. Release the key when you see a small spinning gear appear below the dark gray Apple logo.
2. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities' menu. After Disk Utility loads select the hard drive entry from the left side list (un-dented entry - mfgr.'s ID and drive size.) Click on the Partition tab in the Disk Utility main window. Set the number of partitions to one (1) from the Partitions drop down menu, click on Options button and select GUID, click on OK, then set the format type to MacOS Extended (Journaled), then click on the Apply button.
3. When the formatting has finished quit Disk Utility. Proceed with the OS X installation and follow the directions included with the installer.
4. If you are planning to sell or give your computer away, then do the following: After you reformat your hard drive and reinstall OS X, the computer restarts to a Welcome screen and asks you to choose a country or region. If you want to leave the Mac in an out-of-box state, don't continue with the setup of your system. Instead, press Command-Q to shut down the Mac. When the new owner turns on the Mac, the Setup Assistant will guide them through the setup process.

You need do some recovery efforts to see if you can get it working without having to erase the drive. However, you haven't provided essential information. We need to know what MacBook model you have and what version of OS X that is installed on it. Also, it would be helpful if you could explain what happened just prior to this issue. Please read Writing an effective Apple Support Communities question.