You can simply run one of the below query to check the SQL Server last restart time.

SELECT Dateadd(s, ( ( -1 ) * ( osd.[ms_ticks] / 1000 ) ), Getdate()) AS serverrestartdatetime,
       osd.sqlserver_start_time
FROM   sys.[dm_os_sys_info] osd;  

OR

SELECT name,
       crdate
FROM   sys.sysdatabases
WHERE  name = 'tempdb'