Computer fan would get very loud and the monitor won't turn on

hello I have a windows 7 desktop and the problem is I was using bigger monitor for desktop and I try it for two days but when I decided to my old monitor again I notice that the computer fan would get very loud and the monitor won,t turn on but stay on standby. I don,t know how to fix this or what is the problem because this a first time this has happen since I own my computer. I only had it for 3 months. So can someone provide me with a solution what I need to do. thank you very much