Hi
If you can't do anything because the launcher wont work at all:
Go to a PC. Log into your phones google account on google.com Use the playstore on the web to install another launcher eg Nova or Apex , to your phone. Once it has remotely installed (leave your phone on and connected to data/wifi) you should be able to choose between the system launcher and the new one when you next press the home button. Use the new launcher to go to system settings and clear the cache and data for the stock launcher. Try home again and use the stock launcher (or stick with the new one!)
Hope this helps. If not, perhaps register on the forums and post another question including the android version of your phone. If you post to the forum for your device , Galaxy S5 international, you should get more specific help.
Roger