how to write a email to employees who not inform to HR and leave

Please help me out how to write a email

Hi,
Can you please clarify if you are talking about employee going on leave while employment or leaving the organisation.

Going on leave without informing to any person.

Dear Ms. Tanu,
As a process you need to issue show cause notice to the employee for his unauthorized leave. You may do the same on the day of employee rejoin or you can send it through registered AD. You should make sure that, the employee should not rejoin without giving written reply to the show cause. Depending on the severity of the issue you may decide to issue warning letter or may take further action.

Dear Tanu Bhatnagar,
Firstly is there a practice of issuing a leave application before going on such leave? And leave should be recommended by line supervisor/reporting authority, then sanctioning authority should approve.
If not, introduce it.
And what is the line supervisor/In charge doing if they do go without informing?
It would be chaotic if some one goes without informing superiors.
HRs should only reconcile the leave balances and inform employees of the status. But if unauthorized absenteeism is there HR should be the first one to take a call, as suggested by our senior friend Manoj you need to issue caution memo/show cause etc depending on the case.
If a properly followed leave policy is there, these all come into place, check out and implement.

On first you need to discuss with the employee for which reason he has to leave the organisation.
Second, if the reason was regarding any emergency then you can warn him not to repeat again.
Third, if he has done knowingly then you can share a warning mail, then on the next time you can issue show cause notice.

Whatever the need, your writing must always be courteous. Remember, courtesy begets courtesy!
Leave, and leave related issues, must be part of a very comprehensive 'leave' SoPs, and along with the appointment letter, and related documents, this document must form part of the annexures.
First talk over the issue with the staff for the misdirected absence - could have been an emergency, and was unable to contact the HR or supervisor - you know where both stand. And work out a solution.
For such a situation , some additional clause, can be added to the SoPs, after a detailed dialogue with the management.
All staff must be very clearly informed / educated on these relevant issues.

When you do not know how to write an e-mail, am puzzled, how to respond to you. Everything is available in internet and in CiteHR itself.
Is your problem relate to email relating to absence or writing email itself, as per your first query !
Learn to draft letters, email etc. One cannot afford to be ignorant of simple basic English and come up in an organisation. The younger generation has everything available in internet but there is lack of motivation to search, download, learn and practice. They want everything from seniors - readymade - like Pizza !
Best wishes