Computer screen boots to a black logon screen

Hello Everyone,

When I first start my computer the dell logo pops up and the loading bar comes on but after it is jthere is ust a black screen but my mouse is still there and working. It seems like every thing is working fine cause I can turn all my buttons on and off(like the caps, and built in wireless thing). I also tryed to log in trhough safe mode but still the same thing, help please

thx

(I have a dell inspirion 9400, I had it for quite sometime, this has never happened before, with a vista bussiness op, and....well....is there anything else I also need to say?)