My Arduino is not recognized on the serial port

Hi Guys,
I have an original Arduino Mega Funduino. (2560)
He has worked all the time, I was able to load and run all the programs I wrote.
However - from now on - it is no longer recognized by my LINUX computer (ubuntu).
The TX lamp is permanently on and the On button is yellow.
However, my Arduino is no longer displayed under the - TOOLS - PORT item.

Someybody knows what could be the problem ?

best regards
Celina

Which on button?

Do you have anything connected to pins 0 and 1?

I don't have a Linux setup but does dmesg reveal anything when you connect/disconnect the mega?

sterretje:
Which on button?

Do you have anything connected to pins 0 and 1?

I don't have a Linux setup but does dmesg reveal anything when you connect/disconnect the mega?

I mean the ON LED lamp which is imported in the board, a little bit above the reset button.
I have nothing connected on the board, I just try to write a simple Program "Blink" on the arduino