If you have an aftermarket shifter and the stop is set too far in, not allowing the shifter to get fully into gear then that could be it. Otherwise its a shot syncrho and you need a rebuild.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.