A few weeks ago I decided to upgrade the fork on my mountain bike. Because it was the same model of fork a presumed i could use the same headset that I had on for the original fork. But when i put the fork in, greased it up and tightened the headset, the bars wouldn't move and the headset wont turn at all. I don't know if this because of the crown size or if I need a completely new headset .?!
Sounds like you have adjusted the headset, and the top race has been tightened down too much.
Bearings need a certain amount of tightness (called preload) but not so much they can't turn. Its a fine art to figure out by feel "how tight is just right.
Ideally, your headset should have no play but still be able to rotate.
This appears to be a threadless headset. Adjust by loosening the two pinch bolts clamping the stem to the steerer. Then adjust the top cap's single bolt to provide the right pressure - not too much but also not sloppy.
You should have at least one spacer ring between the frame and the stem.
Lock it in place by tightening the two stem pinch bolts. They're the part that does the real work of holding/locking that preload.
If you had a threaded headset the process would be:
Use a pair of big spanners to back off the lockring, tweak the top race, and hold the race still while tightening the lock ring on top.
Wrap a hand around the top of the headset and rock the bike back and forth while the other hand clamps the front brake on. You should feel none or only the very slightest of play, while still being able to smoothly turn the bars left and right.
Based on your comment above, "so i've been experimenting with different size ones but none of them seem to work," you may need to either switch the old crown race to the new fork. If the lower headtube's are going to be different sizes, the difference is quite dramatic--3/8ths of an inch in diameter difference. You can see it from across the room. I doubt this is the case from the sense I get from your post and comment.
It's best practice to use the crown race that came with the headset. There are subtle (and not so subtle) differences in crown races from different manufacturers. Sometimes what happens with headsets featuring cartridge bearings is that the angle of the bearing race is different than the angle of the crown race (i.e. a 36 degree bearing race and a 45 degree crown race). This mismatch will cause binding of the system before all the play is gone out. With a caged bearing headset, one can get the orientation of the cage incorrect and the cage will bind on the crown race or top centering sleeve when the top cap is tightened enough to remove the play in the system. Typically the open part of the cage faces the cone race (which in a headset is the crown race at the bottom and the centering sleeve above. Check to make sure the caged bearings are facing the correct way.
The one thing that will surely get it right would be to get the appropriate replacement headset and have new headset with crown race that were made for each other. You could even move to a headset with sealed cartridge bearings. These make life easier in a few ways. First the bearings are better sealed against the elements and last longer. Second, cartridge bearings have the races integrated within them. This means there is much less wear on the cups and crown race since there are no longer bearings "racing" on them. The entire system lasts a long time compared to caged bearing headsets. In addition, since the crown race in a cartridge bearing headset doesn't actually have bearing balls on it (it's more of a seat for the cartridge bearing race), one can use a "split crown race"--the crown race has a single split in it's circumference and this makes it extremely easy to press the crown race on the steer tube--no setting with a tube and hammer or a specialty tool you'll use a handful of times in a lifetime. Easy to remove and switch over to a different fork too.
So, double check the orientation of the bearings, while a quick comparison of the two forks steer tube should determine their the same or different (no measuring required--its obvious), try to get the old crown race off the old fork to put on the new one (tricky and can damage both the crown race and fork steer tube). If switching the crown race isn't something you're up for, compare the options of the ones you're trying, and pick the one that most closely resembles the old crown race. This includes not only the angle, but also the height of the tapered part as well. Even the shape and depth of the base can be important. Especially if there is a seal that integrates into the lower cup. A foreign crown race can foul the seal and mess with the bearing preload. You can remove the seal if you continue to have problems with the bearings correct.