Be sure you have fluid in the transmission. If you do, either the gear selector is not reading accurately or the transmission is shot. In that case tow it to a mechanic you trust.
Could be anything!! Get someone to look at it, you wont find an answer here giving such little info. Does the car try to budge but its as if all the powered wheels lock up? Could be anything to do with brakes, gearbox, anything!!
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