Is it possible that you just updated to a new version of AnkiDroid that is required to use a different storage location? If so, your data is still safe and on your phone.
If you go to Settings > Advanced – what is the “AnkiDroid directory” set to? Is it /storage/emulated/0/AnkiDroid
(the old location), or /storage/emulated/0/Android/data/com.ichi2.anki/files/AnkiDroid
(the new location)?
Did you have syncing with AnkiWeb set up and were you fully in sync before you updated AnkiDroid?
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If so: You can sync with AnkiWeb and your collection will be downloaded.
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If not: Here’s the short version of the instructions, but follow the links to the full instructions as well.