There is more information needed! I am guessing you installed as a dual boot, so my question is did the ubuntu "move over" or "install next to" windows? There could be several issues here, did you back up your stuff? There could have been several installation issues, especially if you relied on the algorithms of the Ubuntu to move over your windows system. If it was a fresh install, and your items are backed up, it may be best to restart. If you have a windows cd, it is best done to partition your drive to have 1 ntfs ready partition, leaving the rest (ubuntu space) a raw partition. Then you can install windows, and later install the ubuntu to the raw space on the disc. This is a long but more proper way of installing both linux and windows to one hard drive.
Please post the actions of the computer when you try to boot Ubuntu! : )