Start with the right checks.
First establish whether the file exists on the computer, in Dropbox online or on another person’s device. That comparison can narrow the problem.
What to do next.
Check the Dropbox app status and intended account.
Confirm a working connection and any account storage warning.
Compare one harmless file with its version on dropbox.com.
Record the exact sync error and follow Dropbox’s matching guidance.
Check the result.
Confirm that a test change reaches the intended account and opens correctly on another authorized device.
Stop and get help if…
Permissions, conflicting copies or business network restrictions are involved. Ask before unlinking accounts or weakening firewall settings.