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 [guide|19016|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.[1] That may get you into bootloader.
[1] 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.