My mother has been using the Moto G3 Turbo Edition for almost 2 years now. The device has 16GB of internal memory, but she is using an external Samsung Evo 32GB class 10 memory card as her INTERNAL MEMORY (Thanks to the memory-switching feature of Marshmallow). She has been using the phone with Evo as her internal memory since the phone was first bought. Today while she was using WhatsApp, suddenly she noticed that she was not being able to download any media (music, videos, songs). And when she checked her gallery, she found it empty! Out of nowhere! After further checking it was discovered that the G3 Turbo was not detecting the Samsung Evo at all, eventhough the card was still inside the device. It was showing "No Internal Memory Detected". I tried various solutions like switching off and turning the device back on, cleaning the memory card, clearing cache...nothing works! This has led me to doubt that the problem might be with the SD card. I inserted it into my PC and it wasn't getting detected. The Card has lot of important files in it. What should I do?
Unfortunately, using an SD card as internal (adopted) memory is very hard on the card itself. TBH, I am surprised you got 2 years out of it. Most 3rd Gen Moto devices are lucky to get a year or maybe 18 months and we have seen cards fail in as few as 30 days. The way adopted storage works puts incredible amounts of stress on the card itself with constant read and write operations, even Moto staff on their official support forums recommends NOT using adopted storage if at all possible. SD cards like many types of flash storage have a finite number of write cycles, they will fail, the question is when.
What can you? Reboot and see if it shows up again, then copy it off immediately and replace the card as soon as possible.
Other options are pretty limited and the adoptable-storage tag may give you some options, but in general once adoptable storage gets messed up the data is not recoverable due to the encryption used by Android.
Otherwise, there isn't much UNLESS the phone is already rooted and you are pretty handy with Linux. If it isn't already rooted, then it doesn't matter because of the rooting process requires wiping the device. If you are rooted you need to access the *.key file in /data/misc/vold and you can mount the volume as shown here and attempt normal data recovery in Linux.
BTW, Android has tools to mitigate these situations by maintaining backups in online storage... for example, tools like Google Photo will automatically backup pictures and videos to your Google account for free. What is the old saying? An ounce of prevention is worth a pound of cure.