Ob-Gyn questions? Ask a gynecologist online.
If her pregnancy test via blood test is negative then she's not pregnant. It's very rare to become pregnant after tubal ligation. I would consider the possibility that she's not being truthful.
It's possible to.undo it. A hysterosalpingogram can be done. Dye is injected thru the cervix. X-rays are taken to see if the tubes are blocked.
She would need to want it and consent to it.