|It's a desktop then? I assume it was working before you changed the fan?|
Something happened while you were inside the case. Either you jostled something loose, didn't reassemble it correctly or damaged something, possibly with static.
When you replaced the fan did you also remove the heatsink? If so sometimes the heatsink is stuck pretty tight to the cpu and you may have pulled it out too. If so you need to verify the cpu is firmly in the socket with no bent pins and the heatsink and fan assembly is correctly latched down.
Make sure any add-on cards are firmly seated. Make sure all the power connections are tight.
When working on a computer it's always best to have the power cord disconnected. Not because you might get shocked but because most power supplies provide a small current to the motherboard even when the computer is off and that can sometimes damage things if you work on it under those conditions.