You've not provided much information. However it appears, from your previous post, that you might have been the victim of an assault by your boyfriend (and/or a friend). If so, the Crown Prosecution Service policy is that they will always proceed with a prosecution, even if the victim withdraws their statement, as long as there is sufficient evidence to do so. Further, the original statement can still be read to the court, with the jury being invited to make their own minds up as to whether it represents the truth.
Where somebody makes completely fictitious claims that a crime has been committed, they can be charged with wasting police time. A conviction for this offence usually only results in a non-custodial sentence but a prison sentence remains a possibility.
However, if an actual crime has been committed and a witness to that crime (who might be the victim) withdraws a statement in order to try to prevent the offender(s) from being convicted, that witness can be charged with attempting to pervert the course of justice. The courts regard this as an extremely serious offence, which nearly always results in a prison sentence of at least 12 months (and frequently longer).
Chris