Missing Operating System

[Lindsay]

Hi and sorry to bother i have been in hospital and when i
got out i went to boot up my computer and it came up with
missing operarting system so i reformated the drive as a
friend tried loading linex on system them i put back on
windows. then as soon as i took the operating disk out
and rebooted the same missing operating system came up
yet if i leave the operating disk in it starts up with
windows can someone please help once again sorry to bother