i know this is an old question but i cant help but make an account just so i can tell you how i solved it since no where online says how to fix it. i had the exact issue and went searching for an answer but wasn't satisfied as their resolution is related to groups which my object does not have. make sure that all the vertices are merged together. what happened with me is that I'm trying to make a terrain pond and hills by creating 4 subdivided planes and then combining them to make one big plain (had to delete history to properly remove the groups that are created after the combining action, but that's not the point). i then merged the edges where the edge of the plains meet. however, i seem to miss on a big chunk of them preventing me from seeing the cursor and sculpting.

so in conclusion, if you don't have groups for that object and is in fact one whole mesh like mine, the issue would be the disconnected vertices that need to be merged. it will probably be hard to find where the disconnected edges or vertices are as it is a sculpt. so i would suggest that you just select all of the vertices or edges or faces and then merge. you may have to change the merging threshold.