I have files that are on my SD card and when I click on the file to open it to see what is in the file it says cant open file.

Well it is going to depend on the kind of file it is. The phone is only capable of opening files that have associated apps on there. So if you were to try and open a file that can't be accessed from that type of device (running Android) or missing the application you need to open the file, then it won't work for you.

The first thing you need to do is determine what kind of files they are. What the file type and extension are. Then you will know what app you need to open them through so you can access them.

Now if this is an SD card that was say encoded by being used in a different device, or in that device prior to a reset, then you would not be able to ever access the files.

Another way to try and access the info would be to connect the phone to a computer via USB cord and open the SD card info like you would any folder on the computer. You should have an easier time accessing the files that way. However this does not apply if the files are encoded as mentioned above. Even the computer couldn't access those.
Dec 6, 2018
But what if I could open those files previously? I have the same issue. It seems that I can see all my files on my SD card, but can't open any. Any idea what I can do?