Bluetooth doesn't find a device

First, I’m not a professional user, so I might not understand things that are too technical.

I have the following problem. I was running an audio device via bluetooth. It was working properly at first. But now it doesn’t appear in the list anymore and therefore I cannot connect the device again.
It finds other devices (like my mobile phone) - so I guess bluetooth is working. Just the one audio device I want to use it cannot find.

I already tried to find solutions here and on the net - but nothing that really helped to solve the problem.
My guess is that I accidentally put it on some list where the Bluetooth ignores the device - but that is just a wild guess.

I would be very happy if someone would help me solve this.
Thank you.