Toshiba laptop keeps shutting down/no boot

Hello,

My Toshiba yesterday was working fine when all of a sudden it just shut down. When I turned the power back on i got as far as the select user and the Window Welcome screen but then the screen goes blank and I only see the cursor. I have tried hitting F8 and getting to repair my computer but during the first pass, the computer shuts down. I also tried to do a restore but I get a message that says "restore wizard is already running" and the only option I have is to cancel. Any of the other options like memory diagnostics also shut down during the process. I've removed the battery, held the power key down, plugged in the computer and tried rebooting.. I've tried going to safe mode but can't get to safe mode (at least I don't know if I can because I get the black screen). I've tried F12 but not sure what to do once the options come up. I don't have the computer discs so can anyone help me PLEASE! PS...my system runs Vista