It all seems so simple now. But last night about midnight, I was faced with an inoperative Sig P238 that was brand new and inoperative. To save my fellow travelers angst when they encounter the same issue, here is the 5 minute fix.
The problem part is the ejector, shown here:
It is spring loaded, and you have to rotate it forward in order to put the slide back on after cleaning. The problem is that you can easily rotate it too far forward. When you do that, the back end gets on the wrong side of the leaf spring, and you can't put the ejector back up.
So the first rule is to not rotate the ejector any farther forward than absolutely necessary for it to tuck under the slide when you put the slide back in place.
If you do over rotate, the fix is very easy.
Make sure the firearm is unloaded. That statement seems obligatory, but how you would have a loaded firearm with the slide removed escapes me.
Uncock the action to relieve pressure on the spring.
Remove the left grip.
Now remove the pin at the bottom of the frame. This will allow a small plastic block at the bottom rear of the grip to move down. You want it down about 1/8"-1/4".
With compression on the spring relieved, move the ejector back into its upright position. Make sure the sear is rotated so that it engages the hammer.
Now locate the two-pronged leaf spring at the rear of the grip.
With the flat blade of a screwdriver, press this leaf spring to the rear of the frame. While holding that, slide the small plastic block back into position. Then re-insert the pin you removed.
That should set the situation straight.