Thanks for using JustAnswer. The problem with your toilet not filling after a flush is normally due to a mis adjusted float or fill valve. It could be a clogged toilet fill valve which needs to be repaired or replaced. Information on replacement or adjustment can be found by clicking here.
Click here to watch a video on replacing the fill valve.
Most likely the problem is the fill valve. If no one shut the supply water valve, then the fill valve is the only thing that will cause the rear tank to not fill. You can remove the water supply line that is connected to the fill valve to make sure you have water there, but most likely you do. Always shut off the water before doing any plumbing repairs.