Press and hold the sleep/wake button until the red slider shows up. Drag it, and turn your iPhone off. Press the same button to turn it on.
Navigate to Settings, click on iCloud, and choose Photos. Disable iCloud photo library.
Go to Settings and choose Safari. To clear all the cookies, click on Clear Cookies. In order to clear the cache, click on Clear Cache.
The reason why the video playback is not working might be some faulty app you have installed recently. Make sure to delete possible faulty apps, by finding the app’s icon on the Home screen. Tap on the icon and hold until an X shows up. Click on it to delete the app.
First, back up your files. Go to Settings, click on General > Reset > Erase all content and Settings. You will have to wait a couple of minutes or even an hour, and once the process is over, set up your iPhone as new.