It's "J River".
As you've learned, you shouldn't have done that!
It's important. The version folders within the "J River" folder contain, amongst other things, but most crucially, all of your Library database files.
When you install MC30, it will look in there for an existing library and bring that into MC30 as part of the installation process.
I imagine that when you launched MC after renaming the folder, it made you a new one, with a default library.
Here's what I would do, IF, and only if, all you did was rename the folder from "J River" to "JRiver".
Close MC completely.
Delete (or move) the "J River" folder.
Rename the "JRiver" folder back to "J River".
Relaunch MC.
Is everything now back as it should be?