If you lost the ID card then you have to go to the Regional Centre with the copy of FIR and the demand draft of Rs.200 which is in the favor of IGNOU.
Now, in the case, if you forgot to bring ID card then you have to submit identity proof like Adhaar Card and they will give you supplement ticket of the exam hall and after the exam, they will give you back your identity proof and in next day they will ask for the ID Card.