What happens if I signed a service Agreement in (300Rs Bond paper) and never joined?

I have signed a service Agreement in (300Rs Bond paper) to join a company and work for 2 years. Now I got a better job offer from a different company. Is it legal to not join the company on the Date of Joining (I mean what happens if I signed the agreement and never joined)

If you have received Offer of Appointment from the company and acknowledged receipt of same by signing and returning the duplicate copy to company, you are legally bound to comply with the same.
Service agreement is also a contract between you and the company. If you do not join the company, it will be illegal.
However, you may discuss your plight with the competent authority of the company/HR and
try to find a solution/remedy.

So far as the bond is concerned, the contract created by it comes into operation only on your joining.
Regarding, acceptance of the offer which stands communicated, you can still rescind it, if you are prepared to face legal action, if any that might be instituted by the offeror.