This is what your switch (partially unscrewed) currently looks like if you could see inside the mod:
View attachment 328375
Looking at this picture (with the spring/switch compressed) you can see the exposed threads on the contact pin itself, this is what's causing your issue. Normally this should be screwed all the way down TIGHT to prevent the battery from loosening it as you open/close the mod/change batteries etc
View attachment 328376
These are all the parts of the switch (ignore the non factory spring), take special note of the tiny brass pin, and be careful to not loose it.
View attachment 328377
If you continue to unscrew the switch until the bottom part (button) of the switch comes free, you will likely be left with the middle section of the button assembly sitting inside the mod:
View attachment 328379
Unless its stuck/cross threaded, you should be able to use a fingernail and unscrew it (use the tiny notch for above mentioned pin to help grip) or worst case get a pair of needle nose pliers and grip the interior lip and unscrew it out of the mod.
Then reassemble the switch (tiny pin must be properly oriented in the corresponding notches) and while compressing the spring, use a pair of pliers to snug down the contact pin (silver), this should prevent it from coming loose again due to battery friction/unscrewing.
Hope this will help!
*edit* Ignore the oxgard all over the switch assembly, thats why it looks so dirty.