Tutorial / 500 Internal Server Error

[Dave Stone]

When I try to link to any page of Pericles' tutorial which invokes MapServer
directly (e.g. Example 1-9 onwards), I get:
"Internal Server Error

The server encountered an internal error or misconfiguration and was unable
to complete your request.

Please contact the server administrator, root at localhost and inform them of
the time the error occurred, and anything you might have done that may have
caused the error.

More information about this error may be available in the server error log."

I'm baffled, cos it was working just fine last week. Any ideas?