If you can't access bootloader by holding VOL UP and VOL DOWN and POWER, you may have a broken button. If you're still dealing with this, you might consider following the teardown guide and see if a button is stuck or blocked by some dirt.
Sometimes when a Nexus is stuck in bootloop, I'll hold the POWER button down until the screen goes blank again, and without releasing the POWER button, hold VOL DOWN. That may get you into bootloader.
 Google's information for the Nexus 5 states that bootloader is reached with both VOL UP and VOL DOWN along with power, but my refurbished unit works with only VOL DOWN and POWER.