Can you turn on find my iphone app through computer?

Recently lost my iPhone 5s running iOS 8.1.2 and Find my iPhone was off anyway to turn it back on from another device?

Nope. There's no way of turning it on. You MUST turn it on only using your device, but obviously you can't because, you lost your iPhone.

If you were to lazy to turn it on then just consider it gone and move on. You made it easy for someone else to now have it.