If you had some old DELL or something and you are trying to put a standard motherboard into it then it might not work.
OEM PC makers aren't required to built universal mounts into their cases. They also aren't required to use universal wiring or anything like that.
The OEM PC makers have been known in the past (mostly like 2006 and earlier) to make their components non-standard that way you can't just buy something on the open market and shove it in the PC.
The theory was that if they use non-standard mounts then you have to buy new parts through them and they can charge you an arm and a leg for it.
I used to use an old DELL case myself. The PSU died in it and I ordered a new one only to find the PSU doesn't fit the case. The place where the power cord goes was covered up with metal and a different place was wide open. They wanted to make sure I had to buy a new PSU from Dell if I wanted to keep using the computer.
I beat them by just getting some metal cutters and cutting a huge chunk out of the case. It was ugly as hell, but I could plug in the power cord after I did that. I didn't have to pay stupid DELL prices either.
Anyway, they have been known to do similar things with motherboards too, so you can't connect regular motherboards to their cases correctly.
You may be in that situation now.
If you can't figure out how to get it to fit, you probably just can't.
If so, your options would be to get some tools and modify the case or just to buy a new case.
I would suggest just buying a new case if you find that to be true. A HAF 912 is a really good case and pretty cheap. I don't think any other case has a better price/performance ratio (excepting really good markdown deals).
The good thing about open market cases is that you buy one and you never need to buy another one. You can use it 10 or 20 years into the future just like the day you bought it.
The good cases go a long way toward keeping parts cool and extending the life of your other components so they make up their slightly higher price that way.