I know questions like this have been asked before, but this is a bit different. I told it to install Ubuntu on the Partition that had Windows on it. After that, I couldn't start windows anymore. Can anyone help me?
Indeed your question is different. If you really installed Ubuntu over your Windows partition then your Windows has been overwritten and is deleted.
In case you had non-backuped data on this partition you may be able to restore them (at least partly) with data recovery tools.