Unable to view components after deleting them

I followed the Blinky 555 example and completed the PCB layout course. When I didn’t get the PCB 3D view exactly same as in the course (some components missing), I deleted all the parts and routings one by one until the PCB Layout went plain. I also changed some wiring in Schematic, but when I shift the PCB Layout tab, I’m unable to see any components. Please help me understand how to get the components working and how to automatically updated PCB Layout after any changes in the schematics.