Let's say the following is being returned.

To ssh://git@example.com:7999/path/to/example.git
 ! [rejected]        HEAD -> master (non-fast-forward)
error: failed to push some refs to 'ssh://git@example.com:7999/path/to/example.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Merge the remote changes (e.g. 'git pull')
hint: before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

Try git pull and see if "your local changes would be overwritten by merge". If so, check out my article on resolving your local changes would be overwritten by merge.

If "your local changes would be overwritten by checkout" is being returned, check out my article on resolving your local changes would be overwritten by checkout.