2000 Honda Civic automatic transmission issue

Welcome to the site @JonathanWarren. You either have an easy and cheap issue or a difficult and expensive issue.

The easy issue is that your automatic transmission fluid level is too low for the transmission to operate properly, or it's very old and needs changing. You should check your transmission fluid level and condition. I think on Hondas you typically run the engine until it's hot (this circulates the ATF (Automatic Transmission Fluid) in your transmission and warms it, then shut it off and check the level. For this you need to pull the stick out, clean it, insert it all the way again and then check the level. This needs to be done on level ground. If it's really low you won't even see some on the stick in which case fill it according to the manual, that means the right type and he correct way. If the fluid registers on the stick but it's really dark and viscous then it is really old and not doing its job anymore. In that case a full transmission flush would be a good idea.

Note: if it is low fluid you need to ask why it is low, check for leaks, and keep an eye on the level. If the level drops and you have to fill it up regularly you probably have a seal that needs replacing.

If it's not fluid then you have a mechanical problem with the transmission or torque converter, in which case it's time to ask around for a reputable shop to take it to.

It's quite possible that the shift linkage itself is broken. If you shift into neutral/drive/reverse/whatever (anything but park), is the car free-rolling? ie. can you push it with enough force (and no e-brake). If it's staying in park, then it's likely the linkage. If it's shifting and free-rolling, then GdD's answer (transmission fluid) is likely the culprit.