Replaced motherboard in Dell inspiron 560

ok so about a week ago my system quit working after doing updates it would not come on after several hours. I thought the power supply went out but it did not. I then tested the motherboard it was fried. I bought another replacement motherboard. I replaced it and now when it turns on the power switch is orange and no input but, the fans are running. the only thing I did different was replace the processor with intell Pentium 3.00GHZ. on the list of compatible processors sheet from the dell website for the 560 and 570. it  said that the processor is compatible. I don't know what else to do or think is wrong.