I came across an unusual problem today,
I was using vim, had created a new alias in bash, and wanted to try it out.
So I ctrl-z, exec bash (I do this because I don't like nesting it with bash).
And my job has gone. And I now can't resume vim.
The only option I can think of is to kill vim and start a new session.
vim
ctrl-z
exec bash
fg
Edit: This is independent of using screen/tmux.