No one will tell her why she has a court date, likely because they aren't privy to the details. Your mother's attorney will receive "discovery," which is all of the evidence available in the case. If the case was dropped then it is unlikely that the case can be prosecuted (assuming it is in fact the exact same case) due to either double jeopardy or the statute of limitations. If it is a different case but it is still 20 years old then the staute of limitations would likely still apply in most circumstances.
It is impossible to say exactly what is going on precisely because we don't have access to the police reports, witness statements, and other evidence. Your mother should hire an attorney as soon as possible and start obtaining the evidence.
This is not to be considered legal advice nor does an attorney-client relationship exist.