Black Dell Latitude

Please help me at to solve this problem I have dell latitude lap top?
when my computer starts up the screen is black and tells me we are missing a file
but after one hour goes back to work very slowly.please tell me what to do step by step
Try system restore to previous date then you first noticed problem, if you set up system restore? if not go to my computer and see if you have a restore next to your computer and if its over half full, if so use it. that will restore back to factory settings.
Dell Latitude E6400 first look