'Operating System not found'.
When it boots up on the first screen does it detect the Hard drive?
Can you get into BIOS and see if the Hard drive is detected?
If HDD is detected then you may just have a corrupt file of the HDD or the HDD is corrupted.
Do you have an XP CD? same as in laptop i.e home or Pro.
"Am I likely to have lost everything on there or will it be possible to retrieve it? "
If the HDD is "clunking" on boot up then may be unrecoverable.
If corrupt file then an XP repair will sort it out
If a corrupt HDD then may be recoverable with a repair from recovery consul with the XP CD
Or you could also try and connect the laptop HDD to The Tiny and recover files, What op system on the TINY and is the laptop using the NTFS system.
A little more information need to guide you on the right path.