Lost tabs or different safari s open??

Do you have two or more Safari Windows open? Each window has its own set of tabs.

If you have multiple open windows - and unintentionally switch between them - tabs might appear to go missing, where in fact they are still all open and accessible from another Safari window. Use your App switcher window (e.g., swipe-up from the bottom edge of the screen to the centre - and pause briefly before lifting your finger) to switch between App windows.