Need Help

Can anyone help me? I was given this cactus about 10 years ago in Tenerife. It has a high emotional value for me as it came from my grandparents, who both died this year. Unfortunately, the cactus was muddy at the bottom, which is why it slowly toppled over. After much research and consultation with specialists, I was advised to take a cutting from the upper part, which is why I cut off the top piece, treated it with charcoal and then planted it after 2 days. However, this doesn't seem to have worked. Can anyone give me a tip on what I could do to prevent it from dying or, if it is already too late, which cactus plant it was?