If your phone was running something other than "Marshmallow" before, and you had apps running on the card, then I am assuming you had the phone rooted? Most prior versions cannot run apps from an SD card. However Android "Marshmallow", a.k.a. 6.0 actually can. Unless of course it is a device that the manufacturer has restricted this option on. Sadly, it happens.
If the device is rooted, you would need assistance on that, which I cannot give you. If it wasn't rooted, then I would start to wonder what was done to the phone so that you could run apps from a card?
To run them from the SD card in "Marshmallow" would require the card be formatted and encoded to work. That can't be done while anything is on the card as it will wipe the card in the process. So to make it work again, should the phone not have been rooted, then you would need to move all the apps and other files off the card, follow the instructions for making it work in "Marshmallow" and then move all back to the card.
How to turn external SD card into internal storage on "Marshmallow".
NOTE: Please be sure you start with a blank SD card. It needs to be blank because when the card is converted to "Internal" storage, it will be formatted (wiped) and encoded (making the cards data only readable by that device).
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.