Start off with the easiest items, first check your bushings.
Make sure that neither of your 2 bushings on the cables have popped off up front on top of the transmission, requires you to remove the air cleaner assembly to look back underneath and on top of the transmission.
Then check under your center console to see if the bushing went bad on the linkage.
If it is the bushings, you can get a set pretty cheap, most people use Booger Bushings to replace the stock set.
If your bushings and linkage all looks good, you are probably looking at a broken shift selector. This is a common issue with this transmission, I just had it happen on my built transmission last month.
You will already have your air cleaner assembly pulled, then pull the supply side pipe to the turbo (if you have all aftermarket, it's a different removal process but I'm sure you know how at that point). Remove the battery and battery bracket.
There's a lid on top of the transmission, you'll need to pop the cables off (carefully) before removing all the bolts around the top of the lid. I had to put a jack under my transmission, remove the transmission mount, and lower the entire assembly down about an inch and a half to get enough room to remove the bolts.
Modern has a decent write-up on this:
AGP Billet Gear Shift Selector, 03-05 Neon SRT-4
If the piece falls into the transmission you have to get it out, it will destroy the inside of the transmission or tear up the gears if you don't remove the broken shift selector tab. Best way to do it is to drain your fluid and then fish a magnet down in there and work it out.
It took me about 2 hours to fix, it's not bad, the best replacement part, as stated at Modern, is the billeted AGP shift selector, available through Modern or AGP themselves for about the same price, usually 110-120 dollars.
If the shift selector is good the next step is a broken fork. That's a little more in depth, I'd say start with the two possible causes above (bad bushings, or bad shift selector) then come back and let us know.
