ankitsrdr Your data surely is not lost, you can seek the .bin files in mysql, and restore, just you will need to learn mysql, and never ever do a mysql DOWNgrade since you said UPGRADE from 5.5 to 5.1 that is a downgrade!
In my knowledge the versioning of every software/app goes up, so 5.5; 5.6; 5.7; not viceversa.
Read online about restoring from .bin files. Seek your bin files and give it a try.