SD card has lost its partition table

[clare]

Hi Cel,

Just repeating the story as it happened: I was using 080422 monster booted 
from the SD card, with a Nokia battery in the train reading html files 
with "links". On approaching end of journey, I went to the little section 
power management (?) and selected "dim then lock." I had previously set 
gsm and bluetooth to off. Well I probably left it a bit too long, as it 
was dead when I came back to it. So took out the battery and charged that 
up. But subsequently it won't boot from SD card, just circles round and 
round. saying each time as far as I can tell

Bad MBR sector signature 0.0000
Bad partition,
bad magic number

cfdisk says it has 2G of free space.
I ran dd on it and there seems to be quite a lot of data on it otherwise.

I put in the original SD card and was immediately able to boot from that,
as it had a filesystem I think one of yours dated Nov 30, which does phone 
calls very well.

It worries me though that the DIM then LOCK should have had such a 
dramatic effect. What else might have caused loss of partition table?

thanks
clare