When I turn on the. When I turn on the computer at the mains everything seems as normal, although

Question

When I turn on the computer at the mains everything seems as normal, although when I switch on the hard drive.. the L.E.D flashes red and green, and the monitor L.E.D goes from green to orange instead it should be green. The hard drive sounds like its running as normal. The problem began with the system freezing and crashing, please help