Go to Settings, tap on General, and Usage. Check Battery Percentage, and if it’s set to off, just move the slider to change it.
If you have several icons in the top bar, the battery percentage icon will be removed. As a solution, you can turn off several things such as a portrait orientation lock, location services, alarms, etc. Once you do that, the battery percentage icon will be on its usual place again.
Another solution is to press the Home button on your device twice and to close all the apps. When you do that, press and hold the sleep/wake button, and when the red slider shows up, turn your phone off. When the device boot up again, the battery percentage will hopefully be back.
If there is a new iOS version, make sure to download and install it on your device. As you know, when you do this, certain issues are fixed, which may be the case with this one as well.
If previous options did not help, back up all your data to iCloud, and navigate to Settings, General, Reset, and choose Erase all Content and Settings. In several minutes the task will be done, and your device will get a fresh start.