Why won't my pword work and what do I do?

I tried to log on to my computer today using the same pword I've always had and it suddenly isn't working. I don't know why. I've made sure there is no capslock on, the keyboard seems to be functioning fine. I've even tried every variation of pword I can think of just in case, even possible mistakes or typos that I could've made, which is doubtful since I've typed in this pword a million times. The weird thing is I didn't even forget my pword, it seemed to just stop working. I've restarted, unlpugged, tried to log on in Safe Mode and it just asks for my pword again. Even my password hint is the same. I don't know what to do. I don't know how to reinstall Windows Vista and I don't even think I can...if I do I'll have to lose all my stuff right? This really sucks. I don't understand. Any help would be appreciated.