Please clarify - are you referring to the Android app or our desktop app?
In case of Android, please tell me what version of Android are you using?
In case of the Windows desktop app, please click on the little arrow in the bottom-right corner (in the area where you see your system clock) and see if there's a red TD icon. If there is, it means that Todoist is already up and running, but minimized.
Double-click on this icon or use the keyboard shortcut Ctrl+Alt+T to bring it back.