Two things spring to mind. The entrance might be just buried under dead vegetation?
Compare this photo with the one at the start of the thread. ( not sure if this photo is mine, Lionel Parkinson's or Dave Webb's -my apologies)
The second is, that if it has collapsed only the top few feet may be damaged. I've checked back through the stuff I have and although most of the lining is in that heavy corrugated plastic pipe, the top few feet is made of single skin GRP tube. It could be only this that has collapsed.
edit - although the top is reinforced with a steel ring, that the grill is bolted to (IIRC).