I was using Kdenlive yesterday and it crashed my system.

On restarting and it subsequently crashing a further two times, the system then wanted to download the very updates that I had only installed the day before and VLC had vanished, I just had to install it again.

Now the effects in Kdenlive do not work.

What do I do to sort this out?

I assume you work on Linux. Try with the AppImage 18.12.1b: https://files.kde.org/kdenlive/release/ ... 4.appimage