Dell latitude E6520 startup issue

Hello, i'm just new to posting stuff like this and hoping someone could help me. I've been experiencing a problem with my latitude e6520 laptop for a long time. Still it is working but i have a serious problem when turning it on. When i'm about to turn on the system, the dell logo will appear and the just a few seconds later, it will shut down automatically. I noticed that the memory led blinks while the other led are solid. After pressing the power button again, the dell logo will appear and then a system message will appear, and it said that the windows has failed to start. I keep on encountering this problem when I'm about to turn on the laptop.