I'm assuming that you've already checked the transmission fluid and it's up to level, clean and clear. If the fluid's black and burnt, the thansmission's failed. if the fluid's normal, it's possible there's another cause.
It's possible that the shifter mechanism came apart, There's a shift cable that goes from the shifter to the transmission. It or the shift mechanism could have broken or come disconnected. If the shifter feels normal when you operate it, I'd suspect an internal hydraulic transmission problem. If that's the case, you'll need to take it to a transmission shop ro a Mazda specialist for an opinion.
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