That is usually a client-side error.
If you continue to receive this error message, the issue can most likely be resolved by restarting the Steam client.
If restarting Steam does not resolve the issue, please exit Steam and delete the following files from your Steam directory:
Steam.dll
SteamUI.dll
Rename the ClientRegistry.blob
The files should be found at C:\Program Files\Steam\ (the location on your machine will vary if you specified a different installation folder, or if you have moved your Steam installation).