Hi. 

I have an old forum which I recently updated to the newest version. 

Most things look fine, but it seems like the search bar doesn't work. 
When I try to search for a word it says "0 results". 

Above it is a blue bar saying "The search index is currently processing. Current results may not be complete."

Now, it has stayed like that for a long time. For months I believe.
How can I help the seach index to get to "completion"? If thats how this works?

You need to ensure that tasks are running, otherwise the rebuild will never complete. We would suggest setting this up as a cron job on your server. Please see the following guide on this