I withdrew the retirement plan (ORP) from Fidelity. Fidelity deducted 20% before refunding my investment for the tax purpose. How should I proceed to retrieve the refund?

Since you took a distribution from a retirement plan, you should have received a Form 1099-R that reported the distribution amount and the taxes that were withheld.

When you enter the 1099-R into your return (including the information about the taxes that were withheld) those taxes will be taken into account along with the other income and deductions on your return.  You may not receive a full refund of those taxes, but you will get credit for them on line 64 of your Form 1040. 

If you did not receive a Form 1099-R, then you should contact Fidelity for a replacement copy.