You didn't state from what version to what version you updated. You didn't also state the plug-in you lost. And you didn't state the Computer type and OS version.
Major upgrades are done into their own directory, so third party plug-ins are not carried over and need to be reinstalled. With some plug-ins it's works just by copying the plug-in from the old install place to the new one.
On a Windows pc, you find the plug-ins in the plug-in directory like this: C:\Program Files\Adobe\Adobe Photoshop CC 2017\Plug-ins