My pc starts up normally, until the os logo appears, it gets stuck and all there is, is the flashing dash in the top left corner, occasionally the screen will turn off only to turn back on a split second later, I don't have any flash drives in the pc at boot, the only thing I did before it stopped working was update the os, as it was a year out of date, I want to use my pc, does anyone know what is happening and what I can do?