Need more details. Are you saying when you press the dictation shortcut key (for me, it was set to press the control key twice - I guess you have yours assigned to F5) that you don't get the microphone/dictation symbol (the one that indicates it's "listening" and ready to dictate)? Or are you saying you DO get that, but it fails to "hear" you when you start speaking? I noticed mine was doing the latter, but when I went to System Preferences > Keyboard > Dictation, I noticed that the wrong input source was selected on the left underneath the microphone symbol. I had to change it back to "Internal Microphone". So you may want to check that and also be sure Internal Microphone is set for your input source under System Preferences > Sound > Input.
If you're not getting anything when you press F5, be sure F5 is indeed your assigned shortcut key under System Preferences > Keyboard > Dictation > Shortcut
If all else fails, restart your computer and see if that fixes it.