Work through this in order, because the cheap explanations are also the common ones and people skip them to get to the interesting ones.
The first question is always whether traffic fell or whether measurement changed, and it is astonishing how often it is the second.
Work through it in this order
Did the measurement change?
A tag removed in a redesign, a filter added, a property switched. Check the tracking before you diagnose the site.
Owned by Tracking and attribution
Is it seasonal?
Compare with the same period last year, not last month. HVAC traffic has a shape, and reading a July-to-October decline as a problem is reading the weather.
Owned by Content
Did pages leave the index?
The Search Console Pages report will say. A redesign that changed URLs without redirects does this, and it is usually instant rather than gradual.
Owned by Technical SEO
Did positions fall, or did impressions?
Positions falling is competitive or a content problem. Impressions falling at stable positions usually means demand moved.
Owned by Content
Was it gradual over months?
That is decay rather than an event. The pages did not get worse; what they compete with got better.
Owned by Content
A sudden drop is nearly always technical or measurement. A slow drift is nearly always content. The date shape tells you which before any tool does.