I'm always very careful to keep them the right way round or it can be so easy to get mixed up.
OK then. Something that may or may not work to get them back in. It's already been suggested to try each end in turn and slide them along as sometimes there might be a marginally wider gap at one end.
If that doesn't work, you might want to try the following:- Normally I would slot the top end in and then try to wiggle the bottom end in after. It's natural to try this with the bottom end in front of the runners while the top end is between the runners. Instead of doing it like that, stand the bottom of the pane on the sill, push the top end into the runners, then try to wiggle the bottom end in from behing the runners. Sometimes, this gives you a fraction more to work with as the backs of the runners may sometimes be marginally lower than the fronts. I have actually done this before and it worked - just. Not saying it will work for you but it is another option.
BTW. Leave the swear box at home as you could end up skint