I had a joint account with my niece and she took all the money, I am only trying to get back what I deposited

Your niece stole from you. You can and should make a police report but be prepared to document your ownership of your portion of the money with cancelled checks and bank statements showing your checks deposited into the account. If all your deposits were cash you have a problem- you have to prove you made the deposits You can also sue her civilly for unjust enrichment, depending how long ago this happened.