Check the timeline and see if you only have a segment of the work area selected. You can change this by pushing the "end" button and then "n". That would cause it if you are exporting to .swf.
If you are rendering, in the render settings there is an option under "Time Span" to override the work area and change it to length of comp.
The things John mentioned are definitley the places to start ... if none of those are the issue, you'll want to look at precisely what is happening in your comp at the 25 second mark to figure out what is causing the render to fail.
For example, if that's the frame where a particular layer or effect first comes into play, something about the layer of effect is likely causing the render fail so you'll want to do things like double check effect settings and stacking orders to confirm there isn't some effect parameter set in a way that over-taxes your machine's capabilities (i.e, a zillion particles with high blur) or some similar issue.