What happened ?
Section 39 is common assault, it is a magistrates court only offence. Maximum penalty is 6 months jail , but that be very rare usually a fine or community sentence.
As orderlimit's link says it is normal to proceed with the case in a domestic violence offence even if the victim withdraws their statement . This is because it is VERY common in these cases for the victim to withdraw their statement.
You could be summoned as a witness, depends on if the CPS thinks the case merits it. Without more information about what happened we can't make a guess as to what is likely to happen in your case.
But a word of warning , IF you are summoned as a witness but then refuse to speak you may be treated as a 'hostile witness' . This means that the court assume you are so scared of your attacker that you dare not speak about it. So it will make things worse for the person who attacked you who is then likely get a harsher sentence.
More information please if you want a better answer.