Check context first
Confirm that the viewer is showing Sequence when you expect the assembled film. Verify the playhead is over picture rather than a deliberate gap, and that the picture track is visible. For a source preview, confirm the selected library item.
Inspect the media path
Open Project Status and Manage Media to check originals and derived representations. Reconnect missing drives and resolve mismatched or missing media. If a thumbnail fails, retry it when the source is available; a thumbnail problem does not by itself establish a broken timeline.
Try an appropriate proxy for demanding footage while retaining originals. Note whether the issue affects one source, one sequence region, or everything.
Compare a short delivery
Export a short representative piece and play it independently. If the encoded file is correct, investigate viewer settings and playback conditions. If it shows the same problem, inspect the underlying edit and sources.
Record the app version, macOS version, media properties, viewer representation, and exact timecode when seeking help. Avoid changing several unrelated settings before collecting a reproducible example.