Obviously, I started typing this before there were any responses.
"Do you have a Mac or a PC?"
Most of us who answer here know very little or nothing about Mac computers and their operating system.
Make a partition = a portion of, or all of, the un-allocated (blank of data) space on the physical hard drive is software partitioned - organized using a file system such as NTFS (NT File System) - then that software partitioned portion is Formatted - organized into the minimum sized smaller pieces - allocation units - so that data can be stored on the partition. E.g.Current versions of NTFS partitions have 4kb allocation units - the least amount of space on the partition a file can occupy is 4kb = 4,096 bytes.
Windows 2000 and up can make NTFS partitions - that's the only type you can make while installing Windows or in the Windows operating system itself if the partition's size is larger than exactly 32.0gb (32,768mb). Later versions of Mac operating systems can make and be installed on NTFS partitions.
Whatever you make a partition with, one partition on a physical hard drive must be made ("flagged" as ) Active.
In newer operating systems - e.g. Windows 2000 and up - and newer programs that make partitions, that's done automatically.
In older ones such as gparted ? and Windows ME and previous, making a partition Active is a separate step.
If there is more than one operating system installed on a computer, only one of them can be used at any one time. When the computer is set up properly to dualboot or multiboot operating systems, the operating system partition (e.g. the partition Windows itself was installed on) of the operating system you choose is the only bootable partition on the computer, however many physical hard drives there are.
If you had the dual boot situation working, you selected the Mac operating system one, then you deleted that partition somehow, then you deleted the partition that was "flagged" Active.
Boot the computer using the Ubuntu CD with gparted on it, run gparted and find the setting to make the partition Windows itself was installed on Active.
Making a partition Active doesn't make it bootable unless it has an operating system installed on the partition.