07 RM450 First gear only

http://www.bikebandit.com/oem-parts/2007-suzuki-rm-z450-rm-z450k7/o/m147560#sch509387

This is parts breakdown for your bike.
Look under transmission, then shifter...You will see the exploded parts drawing. This is behind the clutch. Either the spring has come off or one of the dogs may be out of place. If all that is ok, then it will be time to do a teardown to check that the shift forks are installed correctly.

Paw Paw

Thanks, figured you'd be the one to chime in. Praying it's something simple, really don't want to have to tear this thing RIGHT down. I left the manual at the shop that did this, for reference, these guys are really good.

I would try the shift mechanism first before you go splitting the cases again. I bought an RM125 engine with a couple of gears gone. I replaced every worn gear, the drum and shift forks and when I put it back together it would only go into first and then bind.

Finally figured it out. The shift shaft that the lever attaches to, and with the shift paw on it was bent. I replaced that with another one and it shifts perfectly. Also make sure the spring on the back of the shift paw is correctly seated over the pin that is fixed to the case.