Each time you sync your device, iTunes updates a backup copy of your data on your computer's hard disk. If your device has been reset, you may be able to recover your weight history from the backup files.
First, you must download and install a utility that can extract files from an iTunes backup. Here are links to some utilities that may help you; I have not tested them all and cannot recommend one over another.
Use the utility to extract the files for the FatWatch application. Among the extracted files you should find a file named
WeightData.db. This file contains your weight history. Send it as an email attachment to email@example.com and indicate your preferred weight unit (pounds or kilograms). I will be able to convert it into a CSV file that you can import back into FatWatch using Wi-Fi Import/Export.
If you are unable to recover the data from the backup files on your computer, and you also backup your computer's hard disk, you might be able to recover your weight history from an older version of the device backup files. iTunes stores backup files in the following locations:
\Documents and Settings\(username)\Application Data\Apple Computer\MobileSync\Backup\
See also: iPhone and iPod touch: About Backups