Earlier today Valve/Steam apparently had some sort of account-database issue and were forced to restore account data from a backup.
Users have reported missing games from purchases made during the last week.  Another indication of data being restored from a backup is that player's recently changed Steam avatars have reverted to their previous avatar.

I myself am missing at least 8 games from my account and have had my avatar changed.
This error with Steam's record keeping is troubling to say the least.

At the present time I highly recommend you submit a Steam support ticket!
[Steam Support]