First make a complete backup of all current files and folders if you haven't already. You can then do the fix in one of three ways:

Progressively delete suspect files/folders one by one (if you recently updated it, most likely are files such as gupdate.gcd so remove that from .System for sure), then detail map IMG files, GPX, POI, etc.. It's a painstaking job as is the second way because as you remove stuff there's the need to flash original fw back to see if it'll boot, but then load cure fw to try again if it still bootloops.

Remove everything and then add back files/folders progressively starting with empty Garmin folder before loading original fw (it may refuse to boot if there's nothing at all present), see if it'll boot up and if it does (it should) then add least likely files/folders like keyboard, text, voice etc., testing boot each time finally adding the most suspect like detail IMG files in Map and .System folders, etc.. Keep in mind that you can replace detail and TZ maps using GarminExpress anyway, but it won't offer a replacement basemap.

It's worth pointing out that usually permanent/semipermanent maps like basemap and timezone maps aren't the problem however they might be, just like any other file could become corrupted spontaineously. Look for files with unusual symbols or nonsense names too which certainly indicates corruption. If you have any of those and they can't be deleted then re-formatting is your only option.

Finally the third way, is to reformat with RMPrepUSB using only the recommended settings which is what i'd do only if the using one of above methods become too onerous and removing the most likely files hasn't allowed it to boot.

If it still won't boot with just an empty Garmin folder then add back progressively the least-suspect, i.e. Voice, Text etc. folders with their files. However many devices will simply boot completely empty and self-recreate the Garmin folder and its GarminDevice.xml so try that first.

If you have any suspicions as to what might have cause the bricking it'll help narrow down the likely problem file/s. If you just updated firmware it's probably gupdate.gcd. If you updated maps then could be a recently dated .IMG. It could be a Voice or Vehicle file too if you added new ones. If you were navigating at the time it might be GPX.

Good luck.