you connect a real keyboard and mouse (for all former computers, you should get a mouse/keyboard combo, it's quite usefull. Bluetooth ones work fine).
or you edit a pad to keyboard file (.p2k file) where you can assign mouse button or keys to a button on your pad.
See this doc : https://recalbox.gitbook.io/documentation/v/francais/usage-avance/pad-to-keyboard (use a translator, the english version is still in progress)
Thanks @oyyodams . That issue gave me an idea to remove to wifi dongle and for some reason it worked now. Going to keep it off in the meantime until it happens again or until the next update of Recalbox comes out.
@Zing Thanks for your answer. I made a check, it found an error, but it didn't change anything. So I reinstalled from scratch, it rebooted, but after I said the storage is ANYEXTERNAL, same pb.
Then I said to myself maybe it's the USB key which had a pb.
Then I scanned it and it found some corrupted sectors and repaired it, now it works again.