If you want to file using the status married filing separately, you must file before the deadline (15 April). Since your husband has already filed the joint return electronically, you cannot e-file your return. You will need to send a paper copy.
Once the IRS receives both your copy and your husband's return, it will contact both of you because your social security number is on both returns.
Your attorney or you may want to contact the tax preparer to ask the person why the person filed the joint return without your authorization.
If you accept the joint return (by not doing anything), you accept liabilities arising from that return. If the IRS later determines more taxes are due, you will also be liable for the additional taxes.
1. Follow your existing attorney's advice
2. If you didn't sign the joint return, it isn't valid.
3. At the very least, ask for a credit for 1/2 the tax refund in your decree of dissolution.
If you believe that your husband might have lied on the tax return, you should contact a tax attorney as soon as possible about how to file for "innocent spouse" relief.
As one of the previous answers stated, a joint tax return requires the signature of both spouses - your husband does not have the legal right to sign for you unless you have given him a power of attorney. Given that he filed electronically, though, it might be a problem to prove that you did not consent to the electronic signature.
Also, you will need to tread carefully in case there's a possibility that he lied on previous joint tax returns - which you presumably did sign willingly. The IRS could decide to examine past years' returns if it suspects fraud.
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