I own a Samsung ON 7 phone and I was using a class 10 Sandisk memory card which was encrypted by the device in the lock screen and security menu and with no other third party software. Now I have done factory reset on my phone and accidentally forgot to decrypt the SD card, now SD card is detectable and all the files are there and visible as well. but i can't open any file. What to do, please help !
Before you start anything: Did you use the offer of Google Account for storing all your passwords,etc?
If not: Are you a beginner or an Android Crack? If you are a beginner, then it is very, very complicated - get asap to a data recovery firm (very expensive) or get to the National Security Agency :)). As Android Crack I would try ASAP (hope you've rooted the device by a Custom-Bootloader) mirror the whole smartphone (on a phone shell apk with sudo/su then dd if=/dev/mmcblk0 of=/ disk with same mem size of smartphone mem) as well as the SD disk to a PC disk, then try to use a Linux recovery disk with gpart, disktype, testdisk, and e.g. aeskeyfind etc. to recover the data. So far I know the keys are in a certain folder (look on the web/stackoverflow) in the system partition, but if you overwrote the phone with Odin/Heimdall (DO NEVER THAT NOW) then you can forget all data.
The next time after you've rooted/ made a firmware reset and new encryption the first thing you should do is to backup the key stuff on a new Memory card and duplicate the keyfiles as much as you can!
Afterwards everything is hell :)) (Even there are APK which are offering a recovery/decrypting of private encrypted files, I would be very cautious of that - they're mostly fakes/adware and maybe trojans)