Thank you Casacius! My problem is now solved.
On a new machine (PC), I installed Bitcoin; moved the new wallet.dat out of the directory (renamed it and set it aside in a different directory); then placed a backup of my original wallet.dat into the appropriate \Application Data\Bitcoin\ directory.
It took a while for the block count to add up, so I went to bed and left it running. This morning, it was at 129797 blocks and my transaction history was safe, including the recent transaction that I was worried about, which now arrived.
You probably haven't lost them. Shut down bitcoin completely (including making sure the app isn't running but dormant), Save your wallet.dat to a USB drive, delete all files OTHER than wallet.dat from the bitcoin data directory, and run it again. The application does this to a lot of people.
Never delete wallet.dat, never overwrite a backup, always make a new backup of wallet.dat in a new folder anytime you suspect trouble, as well as after any large transaction, for safety. Also do a backup after you spend any part of a large balance, because most of your coins get sent as "change" to yourself at an invisible address and you need a backup of that.