I like these ideas, especially the part about getting the old officers off the account and the new ones on. Do that straight away.
Frankly, I would give them one more chance to respond, then if there really are missing funds (significant), I would bring the police into it. I've been on two nonprofit boards that have a history of someone embezzling funds (prior to my arrival, ha ha!) and the punishment has included repayment of the funds over time. So a line item in each of these budgets has included the monthly payment.
Have you paid a visit to the bank? You need to get the new chair and treasurer on the account, and to get copies of all the bank statements dating back to the beginning of the year -- or longer. That will give you at least a starting point. It's also important to get the previous chair and treasurer off the account.
As far as getting the records from the previous officers, can you enlist the help of the principal? If there's been a dispute, in particular, sometimes it helps to have the principal -- or another neutral party -- intervene and make the request. The financial records and other paperwork are the property of the group. If you haven't, you might make that point. It's not simply a matter of a former officer getting around to passing on "her" records. There's an obligation to return the group's property.
PTO Chairperson resigned in the middle of the school year which left our Co-chair with an unorganized mess to deal with. She is left with cleaning up unfinished fundraiser projects, missing funds, bank statements, etc. Can anyone give suggestion on where we can assist our new PTO Chair to fix the array of problems left from the previous executive committee she has no information and the previous Chair and Treasurer refuse to give any info including any financial information. SHE Has been requesting this information from them for 6weeks and they string her along.