So, like usual then?

If it's a new game, I start off with a basic adventure I always have tucked away. A good starter adventure is a lifesaver sometimes.

If it's an ongoing game, then we probably have stuff we were still doing? Just recycle the prep from last time wherever possible and play for time. "Oh, yes, you have the treasure from the depths of the dungeon, but now your rivals have seized the place and you need to fight your way back out! Totally not just doing this to reuse the dungeon map."

If it's an ongoing game and we just had a good cutoff point? Thank god that player just arrived. Ask them what they're expecting will happen this session, nod sagely at their guesses and work from that. "Oh, you're hoping you'll fight that cult sometime soon? You never know, it might come up sooner than you think!"

Everything else is just good prep advice. Keep generic NPC templates and tokens you can use for anything. Use a whiteboard for any maps you need. Give your players control of the plot so you don't have to come up with it.