Any idea what your pinhole diameter and pinhole-to-film spacing are? The ratio of spacing to pinhole diameter is an f-stop equivalent which generally runs around 200 to 300 in a pinhole camera. That requires exposures that are quite a bit longer than used with lens based cameras, often several seconds or even minutes.

What film did you use? Some films have a high "reciprocity failure" which means making an exposure time twice as long doesn't actually double the effective exposure. With some films, that becomes quite pronounced and can require very long exposures.

Was the emulsion facing the pinhole? Exposing through the antihalation layer on the back of most films greatly attenuates the exposure (not to mention creates a few other problems).

Also, from your comment, I am inclined to assume you got a clear negative -- were there frame numbers and/or film type info showing along the edges of the film? If not, it could be a processing problem.

There's a few thoughts to ponder.

DaveT