I had it installed before this happened. Should I delete it and reinstall?
No, leave Sentinel installed for now.
Try plugging your 436 in with its USB cable and select Mass Storage Mode if it asks you. With that file missing, it may go straight into Mass Storage Mode.
Look at the folder under whatever drive letter windows assigns your SD card after a few seconds.
There will be a sub-folder under the BCDx36HP folder named "firmware". The file your 436 is complaining about should be in that folder and also in the folder that ProScan mentioned. It sounds like your SD card may be corrupt along with the file it is looking for.
Try reading your 436 into Sentinel. Once the read is done, you should be able to do the Clear User Data option from Sentinel's Scanner menu.
Once that is done, you should be able to Write to Scanner and get your favorites back into your 436. I think the Clear User Data may recreate the missing or corrupt file.
If that does not work, then a reinstall of Sentinel may fix things if files are missing from your hard drive.
Once you are back running, I'd get out to a WalMart or something and get yourself a new SD card of 32 GB or smaller and use Sentinel to prepare that card. Your existing card may be failing.
Word of warning, if Sentinel will not read your SD card, you may lose favorites. If the favorites are already in Sentinel, you will still be able to write those to your scanner but any changes you may have made using the scanners keys and menus since you last read it into Sentinel may be lost.
Edit: If you get a message to check your drive for errors after you plug in the USB cable, tell it Yes. If it says it fixed something, try rebooting your 436 again and see if the error cleared.