Depending on your exact Samsung TV model some of these may not be appropriate, but this provides a good list of suggested tips for models from 2014-2018:
1.Select the Exit button on the remote control and your TV should return to the SmartHub. Then try to re-launch the app.
2.Select the Back / Return button on the remote control and your TV should go back to the SmartHub. Then try to re-launch the app.
3.If that doesn't work, select the Power button on the remote control and let the TV turn off, keep holding it until the TV powers back and then let go of the button. Give the TV a minute or two to settle in and then the app usually works.
4.If that doesn't work, go to the Apps section, select My Apps or Downloaded Apps, and then press and hold the Enter button until a pop-up appears and then choose to re-install.
5.If that doesn't work, try SmartHub Diagnostic, Network Diagnostic then unplug your TV. Then re-connect your TV.