If you need to restart Clash for any reason, the best way to do so is to close the app completely and then reopen it. To close the app completely, you can either swipe it away from your recent apps list or force quit it. To force quit an app, you can press and hold the power button on your device until the power menu appears, then tap and hold the app icon until a menu pops up, and then tap the "Force Quit" option. Once the app is closed, simply reopen it and you should be good to go.