The following are the procedures to restart any Android smartphone (even Cupcake):
Step 1: For 3 seconds, press and hold the power button. (In some models, you may also need to hold the 'power + volume' button.)
Step 2: A menu will appear with three options: Power Off, Restart, and SOS or Emergency Mode. Select 'Restart' from the drop-down menu.
Step 3: Your Android smartphone will reboot, and the app not installed issue should be resolved.