HELP: data loss after copying from external disc

hey everybody,
something very strange just happened to me.
I copied a quite large amount of data (80GB) from an external disc to my computer. While the computer was copying (estimated time was about one hour) I left the room. When I returned about one hour later, everything was shut down, because I forgot to charge/plug in the computer. After restarting I found, that the whole external drive was almost empty. And I am not talking about the data I copied. EVERYTHING!
I really have no idea what to do. I checked the trash bin. No, it´s not there.
Does anyone have an idea what occured to me?
Has anyone a good advice how I could maybe recover the data?
It is really important: 7 years of family pictures, most of them have no other backup. That external drive WAS the backup.

Thank you guys in advance for any help!

Try Test Disk: http://www.cgsecurity.org/wiki/TestDisk_Download

It is free.

If unsuccessful try demo of get data back and ifooks promising buy full version

https://www.runtime.org/data-recovery-software.htm

thank you! i will try that now. late reply because of holidays. i will write here whether it worked or not.

testdisk seemed to work fine- I found the missing data. unfortunately the external drive seems to be broken, it is all the time dismounting on its own.
anyhow I managed to save some of my data.
but now i seem to have another problem. when I start the computer after asking for my disk encryption password I get the terminal message: "failed to connect to lvmetad". then I am in the terminal and don´t know what to do with it. the keyboard doesnt react properly either. its whicked! i will post this on a new topic as well.
I could start again with a live usb.
any ideas how to repair this?