1. YOU do not know why it overflowed.
2. Since YOU do not know, how are WE supposed to know what happened?
3. If we do NOT know the problem, how do you propose we tell you how to fix it.
4. If YOU don't know what is wrong, and we CANNOT tell you how to fix it, the only OTHER option is to call a plumber. PERIOD.
5. This is a DIY site, but we do NOT have crystal balls, are not omniscient, or miracle workers. WE do need a little help from the consumer, and/but sometimes the best DIY advice is DO NOT DO IT. Call someone who knows what he/she is doing before you turn a simple repair into a minor, or major, remodel job. You would not be the first person who tried to avoid a $125.00 repair and wound up with a $1,000.00 one by the time he had destroyed whatever he was trying to fix.