If you accidentally delete client from WSUS, you can bring it back by using wuauclt. From client computer you can force registration on WSUS using:
After running this command accidentally deleted client from WSUS, will register again on WSUS and can be located in unassigned computers.
wuauclt /resetauthorization /detectnow
After running this command accidentally deleted client from WSUS, will register again on WSUS and can be located in unassigned computers.