I had a power failure just yesterday. Crashed the computer with everything open.. mail, webpages, everything... Upon turning it back on, same thing as the OP.. Booted to the login, put in the password, and got the cute spinning water loop, I decided to walk away and let it spin. Came back some 1 hour later, and it was back at the login screen so I thought maybe it went into windows desktop and popped back out at the screen saver / blank screen / login prompt... well, put the password in and just the spinning water loop. Decided to do the power off and power on a few times, and same thing, hanging after the password... So, I happen to have beta 7100 still loaded on another drive, so I switch the drives and boot off the beta 7100 with the primary now slave 7127 ... well, it booted fine and I got the 7172 drive open, copied all my files I needed to the 7100 in case of a salvage operation of reinstall completely to the 7127 drive.. after about 20 gigs of copying data, I powered off, switched the drives again, and boom, the primary 7127 worked just fine.
Dont know what fixed with booting it as slave, but fixed the issue without having to reinstall at least, and I have my data backed up on a W7 7100 drive just in case.