If WP-SimpleViewer is no longer showing in the list of plugins, it is likely that some files are in the wrong location so I would delete the existing plugin (manually) and install and upgrade WP-SimpleViewer from scratch.
(1) Log into your web space via FTP and manually delete the 'plugins/wp-simpleviewer/' folder (not the entire 'plugins' folder, just the 'wp-simpleviewer' folder within it).
(2) Download and install WP-SimpleViewer from here.
(3) Upgrade WP-SimpleViewer from SimpleViewer-Standard (which it comes bundled with) to SimpleViewer-Pro following the instructions here.
There will be no need to purchase SimpleViewer-Pro again. If you cannot find your SimpleViewer-Pro zip file, you can download it using the link from your purchase email. If you cannot find your purchase email or your link has expired, then please fill in this Upgrade Request Form to request a new download link.
Please note that WP-SimpleViewer uses the current version of SimpleViewer-Standard (v2.3.0) and must be upgraded to SimpleViewer-Pro v2.1.0 or above. (Versions of SimpleViewer prior of v2.1.0 do not use the 'svcore' folder.)
SimpleViewer Support Team