Why is my Windows partition unwritable from Ubuntu after I defragmented it?

If Linux detects any errors in a NTFS partition (default type these days for partitions made by Windows), then it will prevent you from writing to it in order to prevent any damage to files. The way to fix it is as you found, boot into windows, run chkdsk. Sometimes you may actually have to run chkdsk, reboot, run chkdsk again, for it to clear everything out.