It is important to think ahead about where you are saving your data when working on a GIS project. Due to complex relationships among files in GIS, moving files around can often lead to errors and broken connections. 

Best practices: 

• Maintain a designated GIS folder with subfolders for individual projects.
• If you will be working on multiple computers, store your data on the S drive.
• Save often and then save again. GIS can strain your computer's resources and will likely crash now and then - saving frequently means you won't have to do the same thing twenty times. 
• If you need to submit your data for a class project, follow the professor's instructions on storage from the start. It's easier than moving everything later. 
• If you are storing your data on your computer's hard drive instead of the S drive, remember to back it up periodically. 

To enable available extensions, select Customize from the top menu and click on Extensions. In the dialog box, select the tools you wish to use. If there is a tool you cannot access in ArcToolbox, this might fix the problem.