The DLL error often stems from a partial update. Deleting everything but your games forces a clean, fresh installation of the Steam client without making you re-download 500GB of games.
The "unable to load library steamclient64.dll" error is almost never a sign of hardware failure. In 90% of cases, it is caused by a zealous antivirus deleting the file or a corrupted Steam update. unable to load library steamclient64.dll