Dear …
On __(Date)__, I gave notice of my retirement from __(name of position)__ to be effective __(Date)__. Though I was really looking forward to retirement, things have unexpectedly changed now __(in my life/within the department/etc.)__where retirement from my position is no longer feasible. Present conditions are dictating that I must remain actively employed for a while yet.
Therefore, I am asking you to honor my request to rescind my prior __(notification of/application for/etc.)__ retirement. I can assure you that my dedication to this __(company/department/etc.)__ will not be disturbed in any way by this change of plans. You can expect me to continue giving 100% effort.
I appreciate your consideration in this matter, __(Name)__, and encourage you to contact me at __(phone #)__ if you wish to discuss it with me. I remain hopeful of your decision allowing me to rescind my __(notification/application/etc.)__ for retirement and look forward to hearing from you soon with confirmation.
Sincerely,