almost certainly you will NOT have lost everything, as if the computer is getting as far as the menu you described it implies that the hard disk is still working, it may be possible its got a few bad sectors or similar, so potentially you can of course have lost some data, but its very very unlikely you will have lost much.

If worse comes to the worse you can no doubt get your data back by putting the harddisk into a different PC to copy it off onto a DVD or whatever.