Ubuntu 9.04 will not boot, grey screen and cursor

I am running XP in in my hard drive partitioned with Ubuntu 9.04, I have not upgraded.
On my boot menu I have two options, Windows or Ubuntu.
Windows boots normally. I can also boot windows in safe mode, but not Ubuntu.
If I try to boot Ubuntu after 3-5 seconds the Ubuntu boot goes to a grey/black screen with a blinking cursor in the top left hand corner of the screen and it will stay like that, probably for ever.
I am not very computor savvy but have tried all the otions which flash up very briefly before booting , f2, f11 etc but I have not been able to make any difference to its failure to boot.
I have a lot of information on this OS which I would be loath to lose.
Can anyone help me get back in please.
Thanks in advance for your time.
Cheers ~ Jonathan