Nothing is displayed on the monitor? I will try to help you. Follow these tips:
1) Check the basic stuff. Is everything plugged into the wall in a stable power source? Is your monitor plugged in and turned on? Is the monitor connected and screwed in with the thumbscrews tightly into the tower?
2) When you turn the computer on, does it do anything? Is there power/fans kicking on? Does it look like it's running correctly other than the monitor? If not, you could have something unplugged, make sure you have all the power connectors plugged into the motherboard from your power supply. Typically a 24 prong main connector and then a small 4 prong connector attach to the motherboard.
3) Check all other connections, is everything plugged tight inside the computer on the motherboard? MAKE SURE YOUR RAM IS SEEDED TIGHT, if not this can cause your monitor/computer to not turn on (and I guarantee you everyone here that has ever took apart a computer has had it happen to them their first time).
4) Do you have a graphics card and is it seeded correctly? If you are using a separate graphics card, make sure you plug your monitor cable into it and not the motherboards VGA output. This will not work when a graphics card installs.
5) I'm assuming you literally have nothing on the screen, but if Windows isn't booting, it's because you installed a new motherboard. When you install a new motherboard Windows needs to be re-installed along with it. There's not shortcuts here typically.
6) Still stuck? Start from the tops. Unplug EVERYTHING from the motherboard, no hard drive, no RAM, no DVD drives, no graphics card, just trying booting it raw with the monitor plugged to the motherboard. Does the BIOS screen come up at all? If not, could be a faulty motherboard (or you can try resetting the bios via using the jumpers). If a BIOS reset doesn't work the next thing is....
7) CPU, you had to have installed a CPU into the motherboard with a heatsink/fan. Did you install the CPU correctly? In AMD systems the CPU "pops" in, you just don't lay it inside of the socket. Make sure your CPU is connected correctly or this will cause your system to not boot. Is the CPU fan plugged?
That's eight things I gave you to try to solve the problems. I will add a ninth one... is the motherboard grounded correctly? Metal screws touching metal parts or any loose parts will cause your PC to not want to boot at all. I recommend putting non-conductive/plastic washers inbetween the screws/motherboard. Other than that, your motherboard maybe faulty. Also, I hope you wore a wrist strap or used common sense to prevent static discharge from destroying your parts.
If you think it's the motherboard, RMA the thing, once in a while someone gets a "bad egg" for a computer part that is faulty. Is it brand new? It should work fine. If it's used though you may have gotten a clunker of a motherboard. Good luck, I hope that helps.