First, before you do anything else , check your backups. If you have important files of which you have no backup then boot Ubuntu again copy them.
Next there are a few thing you can do. Start with these:
- The problem
unmountable_boot_volume is often causes by disk corruption: So run checkdisk from the windows DVD (or windows pen drive). Boot, drop to a console with Shift-F10 and run checkdisk.
- Or remove the harddisk, plug it into another windows computer and run checkdisk.(Hopefully needless to say: Booting the windows installation from the other computer.)
If that does not solve the problem you can always restore a backup, or reformat and reinstall, but that is a lot more work.
Also try looking at the log files (e.g. the event viewer logs, or /windows/nbtlog.txt).
On my windows 7 PC that last log looks like this:
Loaded driver \SystemRoot\System32\Drivers\mup.sys
Loaded driver \SystemRoot\System32\drivers\hwpolicy.sys
Loaded driver \SystemRoot\System32\DRIVERS\fvevol.sys
Loaded driver \SystemRoot\system32\DRIVERS\disk.sys
Loaded driver \SystemRoot\system32\DRIVERS\CLASSPNP.SYS <-----
Loaded driver \SystemRoot\system32\DRIVERS\avgrkx64.sys
Loaded driver \SystemRoot\system32\DRIVERS\avgloga.sys
Loaded driver \SystemRoot\system32\DRIVERS\avgmfx64.sys
Loaded driver \SystemRoot\system32\DRIVERS\avgidsha.sys
Loaded driver \SystemRoot\system32\drivers\cdrom.sys
Loaded driver \SystemRoot\System32\Drivers\Nul
It is quite possible that ClassPNP is not the problem, but the files after it is the one locking up your system. (In the example shows that is avgrkx64.sys, part of AVG antivirus. In your case that might be a different file).
Also, did you change anything before your windows stopped booting?
If not, can you add that to the OP?
If you did, please add what was changed.