Press F8 repeatedly at start up black screen to enter start options chose safe mode and back up every thing of importance before doing any thing else.
Note; one should back up all important files on a regular schedule to a external media like CD's, memory card, external hard drive as no one knows when a operating system will crash or a hard drive will fail. Post the brand, model number, installed operating system as all laptops are not the same as how to do a system repair or recovery. You could go to the laptop manufacturer support website look up the user manual by model number and read section on doing a system repair or recovery using the built in system recovery or the recovery disc set you created when setting up the laptop.
Please help me