It's hard to be sure what really happened. I suspect an incomplete upgrade, possibly damaging the bootloader in the process, but that doesn't help you much at the moment. I can answer only the first question (and hence the fourth...) - to some extend. The rest remains to be seen.
Get hold of a live CD to rescue your data (if it's still possible). A very good one with lots of useful tools is System Rescue CD
; but you can use most live CDs - the Fedora
install CD doubles as a live CD. Mount your partitions (if they're not destroyed) and copy your data to an external storage device (USB is easiest - the CD is probably in use). After that, if nothing else works, you can always reinstall Fedora 11
. But if only the bootloader is damaged, it may be enough to reinstall or reconfigure that.
I hope you find a way to get at your data.