My tibialis posterior tendon has come out of its groove behind the medial malleolus is this even possible?

Very difficult: There would have to be a very shallow groove or a very forceful injury.

Anything is possible: This would be very uncommon. The tibialis posterior tendon is held in place by a very broad, strong compartmentalized ligament. It is possible you have ruptured the tendon or ligament. Do you have weakness on that side? Are there any other symptoms. Posterior tibial dysfunction is a serious problem. You should have a professional exam and imaging if necessary.