1. What permissions were removed, are you able to re-add it?
2. Do you see this behavior for only this workflow or all workflows?
3. Do other accounts see the same behavior (k2 service account for example) or other K2 Admin accounts?
4. If it works for your K2 service account for example, perhaps it deal with authorization framework permission or folder permissions:
5. https://help.nintex.com/en-US/nintexautomation/userguide/5.7/Content/AuthorizationFramework/Authorization-Framework-Overview.htm
6. If it works for your K2 service account or other account, you can try adding the non-working account into the ‘Security Administrator’ role.  Perhaps permissions were removed from on the ‘com.K2.System.Management.Form.Process’ System Form or a specific Deny rule was added to the folder containing this System form.

@TinTex  Thanks for reply.

Yes, i removed permission for workflow folder for Everyone to deny and added myself with permission.

Now its not allowing me to access any workflow instance.

For System account it is accessing all instances.

Please attach a screenshot of which folder you are referring to and which permissions.

Are you able to re-add the permissions that were removed?  If explicitly removing permission or denying user access, what you are seeing may be expected.

For that form to load, you may need to allow the users in question permission to ‘View’ it and ‘Execute’ it.

@TinTex 

I removed permission from Management folder under Category → System → Management folder

Everyone → Deny 

And added my self with 2 more users with allow permission. 

Now i revert back permission for Management folder to allow. 

Still me and other users are not able to access any workflow instance using management. 

Using system account i am able to access it.