My BBB disappeared

Hello guys,

I’m looking for help with my BBB.
I’m working on it for a school project.
I recently supplied my BBB with a battery and worked on it with an ethernet link.
But I couldn’t create or modify files with Putty, everytime it wrote me “Read-only file System”.
So I tried to plug my BBB directly on my computer with the usb link but there start my problem.
When I plug my BBB leds light up as usual but my computer doesn’t detect it.
What happens there ?
Can you help me?
Thank you in advance