Cleaned the fan of Compaq Presario c700, now monitor stays black

My laptop had been humming loudly and overheating so I decided to disassemble it and clean the fan, after it was suggested that cleaning might help. Well, I could only get the keyboard off, not the top cover, because of some very firm screws. So I cleaned the fan the best I could with a small brush and put my laptop back together again. I had not been as careful as I had thought, because when I was done, I found this small circular part, one side metal and one side plastic, with a hole in the middle, on the floor, that I have no idea where it came from.

Now when I turn on my laptop, all the lights work and you can hear the start up sound, but the monitor stays black.