Sorry to hear about that and for not answering you earlier.
Did you use Quick Action to merge the videos? You can see how to do it here.
What happens when you try to merge your videos? Do you see an error message? Can you share a screenshot when you do the merge? We recommend trying with a different browser (Safari, Microsoft Edge, Firefox) and clearing the cookies and caches of your browser.
If the issue persists, please share the browser version, the device type and the OS version.