I've had a demo site up for about 3 weeks for people to look at and it's been running fine.
Last night it started delivering blank pages on a contact form page and on the admin page (which were working fine yesterday afternoon).
After reading here, I tried to delete the .js files in _combined files, but I don't have permissions on them to delete them. So I simply renamed the _combinedfiles folder (which I did have permissions on) and all is working again.
What on earth could have caused that to happen suddenly and how can I avoid it in the future?
Okay, I could deactivate combined files in the config file, but turning off useful functions to make the system work is not the point of the exercise.
Its possible the combined_files functionality did not successfully make the files or encountered an error and therefore did not output anything after the error. Disabling it works but like you said its a sledge hammer approach. The best idea is to delete the combined files every time you edit the site and ensure they are kept up to date.
All three files appear to have been created correctly (insofar as they have plenty of content and all close correctly), but as I say, I don't appear to have permissions to delete them on my host, although I can rename the folder. The files are owned by the Apache server and, of course, I can't change the permissions as the FTP user. The files have read permission only for the group, and the folder has read and execute permissions (which is presumably why I can rename it). I guess I could only delete the files programmatically, as that would be done by the Apache server, not the FTP user.
When you say "every time you edit the site", do you mean every time you add content or pages etc. through the CMS? On the final live site, that could be several times a day, and is unrealistic (at least manually - which I can't do anyway, as I don't have permissions).