No need to panic! First...
To protect your project always Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the caret to the right of the Save Project button on the toolbar.
Try an AVI (instead of mp4) export. That often succeeds. Or, in lieu of a purchase...
Reinstall the free version, which may re-enable export.
Install the 'pro' version which even if unlicensed, should be fully featured for the trial period.
If none of those measures succeed come back here.
Discounts are frequent; there may be another one along shortly.