When you buy a new island/area in your theme park, you can’t go back to the previous one, since you sold it in order to manage a better one.
Your previous rides and ingame features will be recovered in a short time, and you will be able to advance further in the game.
You have to start from scratch, but now you will have a faster flow of money thanks to higher multipliers.
To see all the rides available in your current park, go to the upper-right icons (tap on the one furthest to the left) to open the menu. There, you can check which rides are available to build ("Levels" tab) and the actions you need to complete to open a new park ("Parks" tab).