If you notice that one of the Z-axis threaded rods is loose or not correctly seating into the frame of the printer, you can carefully realign the rod and reseat it. Below is a visual guide that will help to walk you through the procedure.
Identify the troublesome Z-axis rod. See image below for reference.
Take the corner with the unseated rod and slightly pull up in order to get the hole above the rod.
Note: Be careful not to pull up on the entire lid which could cause other rods to come out. Just focus on the corner that is loose.
Next, turn the printer on its side in order to get a good view of the hole where the rod needs to be inserted. Take the rod and realign the rod.
Now with the rod aligned, press on the corner of the lid to snap it into place over the rod. The rod should seat well and the lid will snap into place at the bottom of the printer.
Your rod is now back into position. Because this process will probably change the nozzle height, we recommend recalibrating the print bed to ensure the nozzle is at the correct height relative to the print bed.