Hi Mrunalini,
Try to troubleshoot with following steps:
1. Refresh the whole page or clear your browser cache, then open segment form again.
2. Switch to another browser to check whether the issue could be reproduce.
3. Check whether any of your security role has been assigned with sufficient privileges on Segment related entities.
Settings > System > Security
Check assigned security roles to your user account.
> Users > Your user account.
Manage entities privileges of security roles.
(In Custom Entities tab.)

Regards,
Clofly