Contact the bank directly and notify them of the situation.
A checking account may be established with only one signature or with more than one signature on the signature card depending on the bank's policy.
If only one signature is required, any account holder may legally withdraw all the funds or close the account.
If two or more individual signatures are required, but the bank pays the check or closes the account without the needed signatures, the bank may be liable for the loss.
Refer to your deposit account agreement to determine the bank's policy for your account.