I'm guessing you have a manual transmission? Worn bushings on the shifter (while in gear if you can move the shifter side-to-side more then 1/2", they are worn) can cause shifting issues. If this isn't the problem, then you likely have a bent/broken shift fork or a damaged detent plate. Either would require removal of the transmission to fix. If the transmission is out of fluid, this can also cause the issue. On the front of the transmission (if it is a manual) is a 23 mm brass colored bolt. That is the fluid fill bolt. Remove this bolt and stick your finger in pointed downward. If you can't get fluid on your finger, it is low. Add fluid through this hole until it starts to come out.
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I'm guessing you have a manual transmission? Worn bushings on the shifter (while in gear if you can move the shifter side-to-side more then 1/2", they are worn) can cause shifting issues. If this isn't the problem, then you likely have a bent/broken shift fork or a damaged detent plate. Either would require removal of the transmission to fix. If the transmission is out of fluid, this can also cause the issue. On the front of the transmission (if it is a manual) is a 23 mm brass colored bolt. That is the fluid fill bolt. Remove this bolt and stick your finger in pointed downward. If you can't get fluid on your finger, it is low. Add fluid through this hole until it starts to come out.
If it is a stickshift then the shifter is messed up you might have to take it to the shop to have them replace it