Stuck on "Login Window Loading" screen

one night i was putting a big file, about 2-4 gb, into iphoto from my old imac and i forgot to make sure that it stayed on my external hard drive and there wasnt enough room on my main drive because they were small hard drives except for my external. iphoto said there wasnt enough room so i selected all the pics i just transfered and tried to put them back in my external but still in iphoto and then the screen dimmed a little and this message came up saying i need to restart my computer so i just turned it off because i was tired and thought i could deal with this some other time. then a week or 2 later i turned it back on and it got all the way to were it said "login window starting" and stopped about 98 percent. and i waited for a few minutes and decided to just leave it there and clean my room then i watched a movie and when it was done i checked back and it didnt move. whats wrong with it.