If you had correctly set up a backup solution for WhatsApp previously and then later inadvertently deleted something, you should have restored the missing chats/messages from your backups, not using dodgy apps like Dr. Fone
https://howtorecover.me/restore-whatsapp-chat-deleted-messages-android
As for the booting problem, there a few things you can try but it sounds like the Android install itself is corrupted. Anyway,
-- Try starting up in your phone's Safe Mode:
https://www.hardreset.info/devices/samsung/samsung-g930f-galaxy-s7/safe-mode/
When your phone is running in Safe Mode, it's just the Android operating system and no third-party apps/services get pre-loaded during the start up process. If your phone is able to boot up and run in Safe Mode, you at least know some app you've installed previously is causing the boot loop. The task now is to figure out just which one.
-- Try installing Samsung's Smart Switch utility:
https://www.samsung.com/us/smart-switch/
and see if you can restore your S7 into working condition again.
-- the previous two suggestions aren't likely to fix things but worth trying first. When something damages the installed Android operating system to the point where a device won't boot up normally it will typically just boot up into its Download Mode, or in your case, repeatedly just keep rebooting itself. At that point you need to flash a stock ROM to restore the Android OS. With Samsung devices this can often be done using Odin, a Windows utility. Using the Odin utility you 'should' be able to just flash the ROM without also wiping away your saved data, But flashing a ROM is something of a serious endeavor so there's no guarantee.
Go here to download the appropriate ROM for your S7. Be sure to select the correct one and don't substitute, it needs to match your phone model and your carrier:
https://updato.com/firmware-archive-select-model?q=GALAXY+S7&exact=1&r=&v=&rpp=100
Updato is a good source for verified, stock, Samsung ROMs.
Go here for detailed instructions on how to do the flashing process:
https://updato.com/how-to/how-to-install-an-official-samsung-stock-firmware-using-odin
One step you can skip is loading the Samsung driver. During the Smart Switch install it also installs the latest Samsung driver. Don't experiment setting up the Odin utility, follow the directions.
If everything works out your phone should be restored to working condition again. Hopefully there were no internal storage issues during the flashing process and your saved data will still be there. Just note the operating system itself is essentially a clean install so you're probably going to need to do through the same initial setup and authentication process you did for a lot of things when you first setup your S7. (Also, any OTA updates/upgrades you received previously will need to be re-applied.)
