dual boot problem

[squareyes]

Hi all,
I have an HP laptop with win 8 and Ubuntu 1304 AMD 64 bit desktop,
this morning I after a month of no use I tried this morning, to boot win 
8 from grub menu
and got

Error: can't find command 'drivemap'
Error: invalid EFI path
press any key to continue

This puts me back to the grub menu.
Last time I used this machine both systems were working fine. Ubuntu 
13.04 will boot and run ok
only need to fix video driver (amd radeon) but can live with that at the 
moment.
I rarely use win 8, but would be handy if I could use it now and again. 
If I re-install 13.04,
would this be likely to fix the problem?
Any advice would be very much appreciated,
many thanks in advance.
Take care
Winton