Summarising what has been said in comments, with additional info:
When you ran
sudo dd if=/dev/zero of=/devsdb
you created a file in your
/ partition called
devsdb. The command, if not terminated in time, would continue to run until the partition is totally filled up. Since the command was run as root, it would additionally fill up the small safety space that is usually reserved to allow the system to function somewhat normally (about 10% of the partition size, most of the time). This may well have had an adverse effect on your system.
To fix this, I suggested booting the machine into single-user mode (or "rescue mode", or similar) to be able to mount the
/ partition and delete the oversized file.
Your command is unlikely to have caused any other damage than accidentally filling up the partition, so removing this file (and rebooting) should be the only action necessary to fix this issue.