There are many ways you can see when SQL Server last restarted.
The sys.dm_os_sys_info DMV contains a column that holds the startup time for SQL Server. You can run the following command to find the start time.
SELECT sqlserver_start_time FROM sys.dm_os_sys_info
If you open the SQL Server error log in SSMS under Management > SQL Server Logs and scroll to the bottom of the error log you can see when the server was started.
Another option is to use Windows Event Viewer to look for the startup time. If you open Windows Event Viewer and filter your Event Sources (in our case MSSQL$SQL2008) for the SQL Server instance and Event ID 17162 you can find all of the startup times that are still in the Event Viewer log.
Another option is to use the SQL Server Dashboard Report. Right click on the server name in SSMS and select Reports > Server Reports > Server Dashboard and you will get a report