The following is how to replace the front glass on the Galaxy S5 if it is broken. If the digitizer is broken your touch screen will not work correctly and there may even be some black spots on your screen. If the screen is not working at all, then you'll definitely need to replace the digitizer.

Here's what you'll need:

• Amazon.com: Teennitor(TM)Screen Glass Lens Repair Kit Replacement Kit For Sumsung Galaxy S5 i9600 With Tools And Tutorial SHIP FROM USA (Black): Cell Phones & Accessories
• Hair dryer or heat gun (heat gun would be better)
• Gloves
• Lens cleaner cloth

Procedure:

1. Shut the phone off and take out the battery
2. Use the hair dryer(on high setting) or heat gun (on a low setting), and go over the edges of the screen several times to soften the glue
3. Use box cutter to carefully separate the glass from the digitizer by starting at one corner and working your way up - you may need to re-soften the glue or have someone keep the hair dryer on the phone while you pry up the glass. Insert a guitar pick or two when you get one section up. Again, be very careful and patient. If you go too fast you can damage the digitizer or your fingers.
4. Once you get all of the glass off clean the surface of the phone with a lens cloth to remove any dust or residue
5. Make sure the center button is seated correctly
6. If the adhesive came off when removing the old glass apply some to the outer edges of the glass - trim away excess from the parts of the screen that are transparent
7. Align the screen with the phone body and press. It should click into place.
8. Put the battery back in and turn on the phone. If everything works great, if not then something was damaged when removing the original glass and you may have to get a new digitizer (which is much more expensive)

Here's a good video of the process on an SIII (the process is the same on a S5)