​If you have a new enough version, look on the web under "List Management -> Listserv Command".  There is a reindex command near the botton of the screen.

For older versions, at least on Linux, ​I would (on the OS), remove the index files (they are named ".../<listname>.db<various>", and then do a search, as the search will rebuild the index files.    Check with support before you do this, for what works for me may break your production system. ymmv