The right path and deadline depend on what the notification actually says. Identify your situation:
You did not respond to a requerimiento within the deadline (usually 10–15 days), failed to submit a document, or did not pay the fee. The authority declared you «por desistido» and closed the file.
The procedure stalled due to inaction on your part. The authority warned you, and after 3 months of inactivity (art. 95.1 Ley 39/2015) declared caducidad. Note: this does not interrupt the limitation period (art. 95.3).
The authority reviewed your case and decided to refuse. It usually says «se deniega» with stated reasons. More complex, but also appealable.
💡 How to tell: notifications always include a «pie de recurso» — a block at the bottom stating which appeal is available and the deadline. If nothing is stated or it says «pone fin a la vía administrativa» → see the appeals section below.