– Locate Global.mpt in your system (path is C:\Users\<profile_name>\AppData\Roaming\Microsoft\MS Project\14\1033 for Project 2010 client in a Windows 7 system). – Create a backup of Global.mpt with a name such as Global.old (thus you can revert to the original Global.mpt when things go wrong). – Delete Global.mpt
Project will create Global.mpt with the defaults as soon as you start Project again. Note that all the customizations in the original Global.mpt will be lost (you can transfer them to an mpp file before deleting the original Global.mpt).