Yesterday, I was playing Diablo III and I finally reached Act 2 - Caldeum. Then, a friend asked me to join his game to help me level up. He was in Act 5. We played for a while before calling it a night.
Today, I almost Hulk-smashed my PS4 trying to figure out how to go back to where I left off. After hours of getting frustrated, this is what finally helped me (from a PS4 point of view) :
1. Step into a Waypoint and press X. The Map of whichever Act you’re in is displayed.
2. Press R1 to shift the display to the World map.
3. Highlight the area you wish to travel TO by using the left analog stick, then press X to travel there.
This finally returned me to Caldeum, and I was able to play around for a bit, but I still wasn't exactly where I needed to be. It turns out that my friend was playing in Adventure mode, while I was in Campaign mode, and it switched mine too. Although Adventure mode helped me level up quickly, it didn't progress the story.
To fully return to where I had been, I had to press Options, then Quit the game to return to the Main Menu. Under Game Settings, I changed it back to Campaign mode and selected Act 2.
I'm not sure if there are plenty of other noobs out there who have had friends alter their game settings, but if you're like me, this should work.