https://support.microsoft.com/en-us/kb/2919355

Problem:  A fresh installed Windows 2012R2 machine misses Power and Search Button On Start Screen. Tried to run the updates, but it made no differences.

Quick Fix:

1: download and install  : kb2919442

2: Run the Windows update again.  This time, the Windows update will download kb2919355. The Power and Search button will appear after applying this update.