If you want to keep your data,i would first install the TWRP Recovery. Just install the Samsung USB-Driver and Odin on your PC.
Then load the TWRP Recovery file in Odin (for my G900FD twrp-3.0.2-1-klteduos.img.tar works fine, you find the latest TWRP for your device on
https://twrp.me/Devices/) and install the file (Start your device in download-mode, start Odin, connect device to PC and then start installation in Odin). Disconnect your Device, boot into Recovery Mode and you will see the TWRP recovery menu. Press Wipe-button, press advanced wipe, choose cache and Dalvik / Art Cache and wipe the two Caches. In the same menu, then press the button repair data system. Then Press Mount and mount all possible partitions. Dont forget to insert micro-SD Card before because you need it for the backup.Finally press Backup and choose data, system and boot partitions to backup. Restart your device and look whats happen. If the device doesnt boot, start again in Recovery-mode. In advanced settings you find ADB Sideload. Enable it. (In this advanced settings-menu you have also data-manager and terminal access to your phone, if you can use it.). Quit the Recovery-menu and start System again.If the problem persists and your phone is not able to boot correctly, then you have to make a firmware reset, which you can also start from TWRP-menu "Wipe". If this firmware reset works, you can restore all your data easy with the TWRP-backup. If not, you have to install the original firmware from Samsung.
But after you can install TWRP again and restoe your data. (Its also possible to restore your backup with Apps from playstore, but i dont know exactly which Apps are working on a non-rooted device.)