Gunner,

Before starting surgery on the collar, make darn sure the wall wart
charger is working.

More than once I have had a charger, its cable or connector become
intermitent and lead me down the wrong path.

It will have a Nicd battery inside and for the collar to suddenly not
take any charge is unusual.

If you have to go into the collar, a light touch should do it since the
potting material is there to prevent moisture ingress and not for
tampering resistance.

Good luck,

TMT