why Aperture is unable to find the originals, (I've never had this issue in 6 years) and what I need to do to resolve this?
Then we need to know, what happened to the originals. There may be several reasons why Aperture cannor find them. The volume being offline, editing the the originals using an external editor, renaming or moving them, a problem with the drive they are on.
All photos in albums created in the last month have yellow warning triangle symbol
To fix this, you need to point Aperture to the location of your referenced originals using the reconnect dialogue from the "File" menu. "File > Locate Referenced Files"
The procedure is explained in this section of the User Manual: http://documentation.apple.com/en/aperture/usermanual/index.html#chapter=5%26sec tion=16%26tasks=true
Click the paragraph: "Reconnecting Missing or Offline Referenced Images"
Post back, if there still are problems.