After a park has closed, the only way to get to it by bus is from another park, unless buses are still running from the park and you can intercept a bus dropping off at the resort and returning to the park for more Guests, and convince the driver that you left your car parked at the park. Buses run from a closed park to a resort until there are no more Guests returning to that resort, but the later it gets the less likely a bus will be returning to the park.
So if someone was at a resort trying to get to their car at a closed park, and there were no buses still running from the park to the resort, they would need to take a bus to an open park and then get another bus to the closed park. If no parks are open, the only thing I can think of (short of taxi/Uber) would be to get a bus to DS and try to talk their way into getting dropped off at the park (but I don't know what DS Bus Ops policy is on that.)
I want to add that all this is based on my experience as a driver, but I retired 3 years ago. I know some things have changed since then, but AFAIK this is still the way park exits work.