If you can get into recovery mode you may want to try to reboot from there and see what that does. If that does not resolve the problem then from recovery mode you can do a wipe cache partition this could take ten minutes, so just let it do it's thing. This will not cause you to loose any of your data. Then reboot and see if that solvers your problem. It not the go back into recover mode and do a wide data/factory reset, this will cause you to loose data. However, at this point what do you have to loose. Reboot the phone and see what that does for you. I would do all of this with the power plugged in just in case you have a battery issue. If you need help getting your phone into recovery mode see this link: http://forums.androidcentral.com/nexus-5-rooting-roms-hacks/352008-guide-nexus-5-recovery.html