I used the Mercurial collapse command and then attempted a rollback). I now know that this is something that I shouldn't have done.
hg status
abort: working directory has unknown parent 'e1f07eea60bf'!
The working copy contains a reasonably recent version, at least, but I can't commit it.
hg commit
abort: 00changelog.i@e1f07eea60bf: no node!
How would you best recover from this situation? With regards to history, most of it has been pushed to the central repository anyway.