I know of only one way that this can happen in windows, and has nothing to do with Steam other than their not-so-great decision to install all content into Program Files.
If you are on Windows 7 or Windows Vista, and you have UAC set to ON, then change it later to OFF, this can happen.
UAC does thing evil thing where if a program tries to write to a "protected" area like Program Files, EVEN IF YOU ARE ADMINISTRATOR, when you get a UAC prompt and give permission, it actually writes any files that would normally go into that directory into a "virtualized" directory somewhere else.
If you turn off UAC, that will no longer happen, and as an awesome bonus, it will no longer know about the files that it virtualized.
So in your case, if you had UAC on, installed a ton of games, then later got sick of UAC and turned it off, this would happen as you described.
See the second paragraph in "Features" in the article on User Account Control.
If you have done this and turn on UAC again, your content will come back, but be a TOTAL mess because if you've downloaded more stuff with UAC off, then that will be invisible when you turn UAC on, and vice versa.
this drove me a little crazy once before I realized what was happening.
i'm curious to know if this is your issue.
I dug through the relevant support pages and found some information for you.
Short version: Don't accept things you weren't expecting from people you don't know.
Long version:
The Steam Support page on trading and gifts says...
[...] a gift sent to your account has been revoked. The most common reasons this will occur are:
- There has been a purchase error with the game you were gifted
- The purchaser has filed a dispute over the purchase
- The gift purchase was made using a fraudulent payment method
If you believe there has been an error, please contact the person who sent this gift.
The subscription to the gifted game is no longer valid. If you wish to continue playing the game without losing any game progress, you will need to purchase the game through Steam.
Warning: Never accept a gift from someone you do not know.
Meanwhile, a few pages away...
Redeeming Fraudulent Gifts
Never accept a gift from an unknown user. Any accounts tied to a redeemed gift from a
fraudulent source may be suspended.
In summary: You can either lose the game you were gifted, or possibly your account, depending on the whim of the Steam Gods handling your case. Expect the worst, because they tell you clearly in both areas don't accept things you weren't expecting from people you don't know.
Ask the person trying to gift or trade you the game to identify themselves, and reject the gift if anything seems suspicious. Better to lose a "free" game than your account, ne? If it turns out to be a friend or just someone who feels like gifting you stuff, ask them to send it through their real account, instead.
Best Answer
Here are some possibilities:
If those are not the case, have a look at this Steam guide for more options.