I want to make sure you're able to track miles without any issues. You can first try looking up the app list on your mobile device and see if it is still installed in there. There are also times that the app might be grouped on another folder. Restarting your phone can also help.
If you can't find it, you may get the QuickBooks Online mobile app in the App Store or from Google Play. This is if you're using QBO as your accounting program.
Otherwise, if you have the QuickBooks Self-Employed program, here is where you can download the app: