Please help! my front camera has stopped working. I have tried everything (clearing cache, factory r

The one thing you haven't tried (because you most likely can't) is replacing the camera module. It could be a hardware failure.
 
It's most likely a hardware issue if you did a factory reset, and not all that uncommon with the Nexus 5 (happened to my friend's). LG fixed it though so you may want to contact them. Oh, first try disabling trusted face on smart unlock if you have that on. That always makes my front camera crash