Logged into Steam to find that all of my games on the D: drive weren't installed; games are still in the folder
Solution 1:
I have Steam Library split onto 3 different partitions and unfortunately it happens that, due to some unknown bugs, sometimes Steam starts and does find only default Steam Library.
But you can easily fix this problem manually.
Go to Steam Settings -> Downloads
Press Steam Library Folders button
Press Add Library Folder and select the folder on your hard drive where you had installed the additional games.
I suggest you then to close Steam and relaunch it.