Hello Sunita,
Eligibility Criteria for admission to MCA in IGNOU is-
Candidate should have any Bachelor’s degree of minimum 3 (three) year duration from a recognized University and Mathematics as one of the subject at 10+2 level or graduation level; else the student is required to pursue and pass the BCS12 course  concurrently  with IGNOU  MCA.
You have completed your PG Diploma but I don't think that lateral entry is an option for admission in IGNOU. However, you should contact the authorities to clarify the same.
Good Luck!