You could have fried something with static electricity when installing the fan. Doesn't happen very often but a potential problem.
You could have also unknowingly moved or yanked at one of the wires, and now things won't start up without it.
But this is all guesswork and hard to troubleshoot unless you start crossing things off of a list. PSU first else any other troubleshooting is impossible if you can't even get to bios or get any error beeps. Once you have that working, you check the mobo, replace the cmos battery (to reset bios), try and see if it boots at all, if any error beeps (these can indicate memory, cpu etc). Lights on mobo help as well if you've got a mobo manual to tell you what they mean. What color, are they flashing, manual will tell you how to interpret them.