My S5 started doing this also yesterday. I also noticed that google search app updated to google assistant. I think that is where the issue lies, cause it's now working fine on my phone
Here's what I did:
Go to Settings>scroll down to Language and Input>Google Voice typing>"Ok Google" detection from there, follow the instructions for setting google to recognize your voice and make surethe "on/off" toggle is blue. That's it! Hope it works for you.
Collette