Hello Neelam yadav,
Could you clarify whether Word itself will not open at all, or whether Word opens normally and this error occurs only when you try to open a particular document?
If Word itself will not open, the Online Repair already suggested is a reasonable next step.
If Word opens normally but only one document fails, please mention that as well, since the troubleshooting would be different and may need to focus on the document itself rather than the Office installation.
If possible, please also share the complete error message or a screenshot of it, with any personal information removed.