You will not be able to play Fire Emblem Heroes again unless you reinstall the app. Additionally, unless you linked the app to a Nintendo Account before it was uninstalled, all of your data (including your Orbs) will be lost, and you will have to start again from the beginning of the game after reinstalling the app.
Before you uninstall the app, please confirm that you have linked it to a Nintendo Account.
Please see this article for help linking your Nintendo Account: How can I link Fire Emblem Heroes to a Nintendo Account?
Please see this article for help checking your app's link status: Can I check which Nintendo Account is linked to my app?
Also, if you intend to switch to a different device, please see this article: How do I transfer my save data? (When changing to a new device, etc.)