There are a few reasons that could cause a transmission to stay in neutral. One reason could be that the shifter has become disconnected from the transmission. Another possibility is that there is an issue with the shift cable or linkage. It's also possible that there is a problem with the transmission itself, such as a broken gear or damaged bearings.
If your transmission is staying in neutral, it's best to take it to a mechanic for diagnosis and repair. They will be able to pinpoint the exact problem and get your car back on the road in no time.