Solution for Windows
To fix this error use the following steps:
1) Exit Steam
2) Navigate to your Steam installation directory. (C:\Program Files\Steam\ by default.)
3) Delete everything in that directory EXCEPT the following:
steam.exe
steamapps directory (All of Steam's game files are kept here.)
4) Restart Steam to test the issue
Cause
This error results from a corrupted ClientRegistry.blob file, typically caused by Steam unexpectedly closing. This is often due to a crash or because the Steam.exe process was killed before Steam finished writing to the file.
Some instances of this error can be avoided by allowing Steam to fully exit by clicking Steam ▸ Exit or by right-clicking on the Steam icon in the System tray and clicking Exit, rather than forcing the process to end.
Source:
https://support.steampowered.com/kb_article.php?ref=3093-MKUJ-5773