Strange behavior. Last night my computer got a black screen in the middle of me doing something

Question

Last night my computer got a black screen in the middle of me doing something then began acting odd. When I turned it back on the yellow light in the front (the "thinking" light) began flashing rapidly. It did this a while then the computer came back up as usual. A while later it turned black again and the light began to flash. Ever since then the comp will start up normally once in a great while but mostly it just flashed the yellow light at me rapidly. I unplugged it when I went to bed and it stayed off all night. This morning I plugged it in and it turned it's self on and is still flashing. It starts out rapid then gets slower and slower till it seems to shut it's self off then turns it's self back on and does it all again. It ignores the power button and only unplugging it makes it stop. Any theories? I am very confused at this strange behavior.