troubleshooting: mention missing files

Co-authored-by: Jorge Orpinel <[email protected]>

Fixes #1660

content/docs/user-guide/troubleshooting.md

@@ -7,6 +7,14 @@ This file uses a special engine feature for the following headers, so that a
 custom anchor link is used. Just add {#custom-anchor} after each title:
 -->
 
+## Failed to pull data from the cloud {#missing-files}
+
+Users may encounter errors when running `dvc pull` and `dvc fetch`, like
+`WARNING: Cache 'xxxx' not found.` or `ERROR: failed to pull data from the cloud`.
+The most common cause is changes pushed to Git without the corresponding
+data being uploaded to the [DVC remote](/doc/command-reference/remote).
+Make sure to `dvc push` from the original <abbr>project</abbr>, and try again.
+
 ## Too many open files error {#many-files}
 
 A known problem some users run into with the `dvc pull`, `dvc fetch` and