I have been scanning the forums regarding the S5 Android 5.0 update disaster. Here is a fix.
1. Download S5 firmware from www.sammobile.com for Android 4.4.2.
2. The zip file includes Odin flash program.
3. Put the phone into download mode. Volume button below;home and power button.
4. Fire up Odin
5. Look for the md5 file in the package.
6. Put the md5 file in the AP field.
7. After checking start the flash.
8. After flashing take USB cable out of the phone and wait for reboot.
I hope that this resolves your issues. It may be an idea to enter recovery and delete cache partition and delete data. This way it is wiping everything.