Are you saying the whole PTO disbanded, or just the board? Either way, the treasurer cannot withhold funds indefinitely. Have you escalated this to your principal and/or district? It's definitely time to do so.
Our entire board from last year disbanded and now we have no access to the funds raised. The treasurer is being non responsive about any of balances and funds raised and refusing to write a check putting the funds into the new account. She also says legally she doesn't have to divulge any information or produce the funds. This all happened because the records were being questioned and the entire board closed ranks instead of proving there wasn't any funny business. What do we do now?
I agree with PTOSunshine that the PTA council can help, and maybe they have procedures to handle these sorts of things, because (as I understand it), they have some responsibility for the groups they oversee.
On the other hand, I think the principal could be a helpful ally: the principal is local and can probably get the treasurer to come to a meeting, or ask her to step into the office before/after school -- something the council officer can't. The principal is not in charge, but I would think that the PTA is an important enough source of funding for the school that the principal would be concerned and willing to spend a little time trying to sort things out.
When you say "no longer allowed to make any financial transactions" does that mean that the account was frozen by the PTA council? If so, great; otherwise, take MIDad's advise. For some banks, any signatory can come in and (after identifying him/herself) change the other signatories.
The other possibility is that the treasurer is really in over her head and is embarrassed to admit it, and can't answer your e-mail without admitting there's a problem. Is this a new treasurer, and a different one failed to file the 990?