Your negative test does not conclusively rule out infection due to COVID-19, but means that it is more likely that your symptoms were due to a different infection which could be passed on to others, such as influenza or other bugs that cause coughs and colds. It is important that you do not pass on these infections either.
Therefore, if you work with vulnerable people, you should not return to work until 7 days from symptom onset, and only then if:
If you do not routinely work with vulnerable people, you may return to work if you do not have a high temperature (without taking medications such as paracetamol, etc.) and are medically fit to do so. If you become unwell again, you should self-isolate again, and may need a further test.
If you have had a NEGATIVE test result but have never had any symptoms, there is no need to self-isolate. If you subsequently become unwell, you should self-isolate, and may need a further test.
Please note that you should inform your employer of your result as soon as possible after receiving it. Please liaise with your employer regarding return to work or sick leave as appropriate. This may include occupational health assessment.