I am sorry to learn about this situation. It appears to be a legal malpractice issue with your law firm (as you note, this is the law firm's issue regardless as to whether or not the specific attorney that was representing you left them or not).
Legal malpractice cases are based almost entirely on expert testimony. You must not only find an attorney to act as an expert witness regarding the standard of care for your prior lawyer (to prove the "negligence" elements of the case), but also an attorney expert (and potentially other experts) to testify regarding the underlying claim (you must prove that absent the attorney's malpractice you would have achieved a better result - often termed "the case within the case").
Due to the complexity of these claims, I highly recommend that you retain a legal malpractice plaintiff's lawyer (also called trial attorneys). Fortunately the majority of these attorneys will provide you with a free consultation, and many will represent you on a contingency basis (they will advance the costs of litigation and legal services in exchange for a portion of your successful settlement or judgment).
You can find local attorneys using the State and Local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).
Short of filing a lawsuit, (and perhaps more effective in a situation such as yours) you can try to mediate the dispute with them - contact your local bar association and request referrals to mediators, a third party neutral can often help you reach a mutually agreeable resolution. Use the bar association's referrals to contact a mediator or two, the mediator will then contact the other party to set up a mediation session, and you can go from there - hopefully resulting in a formal or written settlement agreement, and save yourself the time and expense of litigation.