Do you have credits in the machine? Would the bill's credit take you over the credit limit? If that is not the case, you may have to re-calibrate the validator or clean the heads in the validator. If the validator is off or disabled, it wouldn't even take the bill.
You can replace it with another DBV200 head that is known to be working, or you could change the entire validator for a WBA12/13, which are more modern and usually less prone to issues. The WBA12/13's latest available firmware will also take all new bills except the new $100. Replacing the validator involves getting a new validator cage, power supply, and validator harness. In my opinion it is worth upgrading to the WBA series.