Q: Yosemite login screen problem

I am using Yosemite and have a single administrator user account and guest user enabled. Normally, my login screen would be a grey background with the options of the main and guest user selection enabled. However, a while ago while booting, my computer initiated a second boot –after playing the startup sound and showing the apple logo– and since then the login screen is a blurred background of the Yosemite photo with only the main (administrator) user showing, though the guest user is still enabled in the settings. Would anybody have an idea as to what is happening?

Mid-2015 Retina MacBook Pro, OS X 10.10.4