If the girl was not ready yet to answer yes, she can say no. Having your boyfriend proposed to you is such a good indication that he was dead serious in relationship with you and wanted to put his relationship to you to the next level. But if you aren't ready yet, you can say no. This could result to a disappointment or depression to your lover but this does not mean break up or the end of the world.
If you would say yes, the wedding and marriage would be planned. It may be planned early or even 5 years to wait. If you would say no, just say it with out ever making your boyfriend feel ashamed about it as he might not want to propose again to you in the future due to fear of feeling ashamed again. Just be with him, if you still love him. And if you no longer love him, be honest.