If someone is accused of sexual assault, cps and law enforcement has been involved, cps has now closed case, there is no evidence but the law enforcement wont drop the case.

 
CPS and law enforcement (at least hypothetically) are two different entities with different tasks and standards.
 
CPS's goal is to see if a child is in danger and if a family is in need of assistance.  (again...hypothetically)  They will usually make one of three findings:  'reason to believe', 'reason not to believe,' or 'reason not to believe.'   If they have a reason to believe something happened... they may get involved to offer services.  However, even if they believe that something did happen, if a child is otherwise safe, they may chose not to get involved because the child is in a good place and subsequently close their case.  If they are unable to determine, they will usually offer minimal services, if any.  If they determine that an event did not happen, they will often close a case and not offer services. 
 
If CPS closed these case for because they entered a finding of 'reason not to believe' or 'unable to determine', then either finding can be used in the defense of the criminal case.  However, the CPS case will not control the criminal prosecution.  Only the agency and the prosecutor can make the decision to terminate the criminal prosecution.  It may not be closed yet simply because it hasn't worked through the system yet.  Many prosecutors will run a case like this through the grand jury process.  To get to the grand jury, a case has to be delivered to the DA's office, it has to go through intake and be reviewed by and attorney, and then presented.  Some DA's office move quickly....others are so understaffed that it could take several months.  So... the closure just may be a waiting game. 
 
If you get the feel that law enforcement is continuing the case despite a 'reason not to believe,' you need to have an attorney lined up to assist you.  Some DA's office will allow defense attorneys to present grand jury packets--- basically---a packet to tell the grand jury why you didn't do the acts alleged.  Some DA's offices are afraid of the truth finding function of grand jury and will not allow grand jury packets... but it is worth a shot if you have a decent DA's office and the case is still moving forward.  If you cannot afford an attorney at this juncture, then you can request a court appointed attorney after you are formally charged.