If you have e-filed your return and you do not receive your tax refund within a reasonable time you should check if ITR-V of return is verified or not. (you may check it on our mobile app for free). Return will not be processed till ITR-V is verified.
In case ITR-V is already verified, but still there is delay, you can follow up with the Centralized Processing Centre (CPC) of the Income-tax Department on 1800 4250 0025 or 080-2650 0025.
If your return pertains to previous years, where you had filed your return manually then you can write a letter to the Jurisdictional Assessing Officer. Also, attach your Income tax return copies with such letter.