The best UX would be letting the user decide if they want to pick up where they left off, or start over.
Sometimes people abandon forms on purpose, because they don't like what they've written. Other times they fully intend to complete the form, but get distracted.
When the user logs in again, you can do a few things:
- Take them back to the form they were working on with all the fields they filled in fully intact, but make it easy to discard the form if they no longer want to fill it out;
- Take them back to the form and ask in a modal if they'd like to keep working on it, or discard it;
- Take them to the usual place they go after logging in, but allow them to recover a draft state of the form and keep working on it if they'd like.