kafka logs gone after reboot the server

Hi, All

I've just notice one thing, when I am experiencing some errors in Kafka
servers, I reboot the dev servers (not a good way), after reboot, I get
into zkCli, I can see all the topics still exist. But when I get into kafka
log directory, I found all data gone, see

root [ at ] DO-mq-dev:/tmp/kafka-logs-1/ui_test_topic_4-0# ll
total 8
drwxr-xr-x 2 root root 4096 Jan 2 09:39 ./
drwxr-xr-x 46 root root 4096 Jan 2 10:46 ../
-rw-r--r-- 1 root root 10485760 Jan 2 09:39 00000000000000000000.index
-rw-r--r-- 1 root root 0 Jan 2 09:39 00000000000000000000.log

I wonder, if for some reasons, the server down, and restart it, all the
data in hard drive will be gone?

thanks

Alec Li