Sorry to hear that your video player is not showing on your website. Thats not fun.
The good news is, there is an easy to understand cause and an even easier fix.
If you are using any form of caching or performance plugins on your website, they can be causing this issue.
Likewise, of you are using a CDN, content delivery network, this can also cause this issue.
So for starters, there is a simple setting you can toggle off in Presto Player in order to see if this is the cause.
Simple visit the Presto Player settings panel > click on the performance tab > toggle both options off > save changes.
Now test your videos to see if they are working.
If they are working now, then that means there is some form of caching or CDN on your website that is causing the issues. You simply need to exclude the Presto Player JS file as outlined in this doc.