Mac login not working after initial screen Suddenly my MacBook Air won't accept my password.  When I restart, the first screen offers to let me login with my username and symbol, and accepts my password, but then opens a screen giving me the option to logon as Guest or a fill in the blank Name, and if I put on my name and password it doesn't work. I can't get past this except as guest so can't get in the Apple menu. Any idea what is going on and what I can do ? Have tried restarting, turning off, logging in as guest, and still locked out.
May be the trash became full and not enough memory to bootup. You try boot in target mode , mount the disk in another mac machine and clear the trash.
or 
In Recovery mode open "Disk utility -> repair disk" may help.