USMLE is for foreign certified doctors to get a US license. I am assuming that failing the test affects your immigration status. I recommend that you speak to an immigration attorney.
As to failing the test due to computer problems, I don't think you would be able to sue the USMLE board, since the computers presumably worked for the testing purposes. If their equipment failed, then it is likely you would need to appeal to them and make a final determination that they would not let you retake before any lawsuit could be brought. However, I fail to see what remedy a lawsuit would bring you. What cause of action would you claim? That would determine what kind of attorney you would need.
In this case, I do not see any.
In any case, the tests are administered by prometric, and USMLE tells them how to handle such scenarios, so have you tried contacting USMLE and asking them what they can do about it? Your first step should not be to sue them.
This does not constitute legal advice or the engagement of my services as an attorney.