is remarriage after mandir marriage illegal?

As from the facts following things are not clear :

1. What was your age when you got married in the temple?

2. Whether a marriage certificate was issued by the priest conducting the marriage or not?

3. Whether marriage was consummated or not?

4. Where there any witnesses to prove that you got married?

If you can provide the above information then we could be able to guide you perfectly.

Your ex boyfriend has the liability to prove in court that he was married to you

But with the limited information provided by you we would like to tell you that,

If a marriage certificate was provided to you by the priest and there were witnesses to prove that the marriage happened between you two then your latter marriage would be considered illegal. 

And your ex-boyfriend could file a petition for restitution of conjugal rights under section 9 of Hindu Marriage act.

Section 9 of the Hindu Marriage Act 1955 has been explained below for your understanding:

Section 9. 

Restitution of conjugal rights - When either the husband or the wife has, without reasonable excuse, withdrawn from the society of the other, the aggrieved party may apply, by petition to the district court, for restitution of conjugal rights and the court, on being satisfied of the truth of the statements made in such petition and that there is no legal ground why the application should not be granted, may decree restitution of conjugal rights accordingly.

But this can be avoided if you file for divorce. 

But if no certificate was provided to you and there were no witnesses then only your latter marriage is valid and the previous marriage is invalid and your ex boyfriend could not cause any trouble.