I think a simple letter will do.
Dear sir/colleague,
As you are aware due to 'Lockdown' on a/c of impacted by 'CORONA-19 spread in the country imposed by authorities our office/industry/Unit/ Project could not be operated in the usual manner. Many/a few of us were authorised to 'work-from-home'.
Now the 'Lockdown' has been relaxed subject to all of us observing certain safe measures both within our premises and outside the premises offices/units are permitted to open and conduct regular operations w.e.f. . Following this instructions all are called back to report for duty in their respective departments/sites/projects as the case may be with effect from/immediate effect.
Please send us an eMail/letter, 'report to duty' confirming your reporting. Failure of which will be treated as 'absent' without intimation and would be proceeded with as per rules.
Those who are on leave should revalid the same by a fresh request explaining the reason therefor.
Difficulties, if any, for not adhering to the above instructions should be brought to our notice in writing/mail without any delay.
...
HR/HOD.
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You may/are free to modify to suit your requirement.