Suddenly I can't boot in to Ubuntu
does not really exist...unless it happens after something else happened. What did you do? Updates, installs and so on beore this happened?
Could it be that your Grub does not have access to the Linux partition anymore but still access to the windows? Fixing Grub requires some insight into the menu list. Try to pub that here...
If you cannot boot that deep, try TTY Linux
to boot the box...
Stuck? Let us know...