I was doing my development work in a sandbox which got inactive. Is there any way I can activate it again or get my data from that sandbox account?

as in your sandbox agreement with netsuite expired? Or someone else removed your user access? Or you/someone activated a refresh and now all the data/development work has been overridden?

I believe inactive accounts are purged after 30 or 60 days. You would need to reach out to NetSuite support to see if you could get access to it again. Would also need approval from the account owner too

None of the users can access the sandbox account which has been inactivated.

sounds like you need to call netsuite support and/or your account rep to get them to re-activate it. may need to negotiate terms if your contract for that has expired.

sorry to hear. trust no one, always backup.