Hey @MM_Panju, What kind of device is it? Do you see the files when you login to Dropbox web? Can you please make sure that you have loged into the app using same email and you do not have multiple Dropbox account?
Is your iPhone connected to the internet? Note that the Dropbox mobile app does not sync and store all your Dropbox data locally on the phone. You will be able to browse and access your files when the phone is connected to the internet. If you want to make certain files and folders offline, you have to specifically make them available offline from the options menu.