Is this NY? For the state, they only care that you take and score whatever is necessary for the diploma and endorsements/designations you want…when doesn’t matter much. You can sit for it in August or January.
However, you mention your district uses it as a final counting for 10%. What happens with your final grade is up to your school administration and how they handle it will likely depend on the policies they have in place, why you missed it, etc.
No colleges really care about the Regents. Most states don’t have them. So, don’t stress about that part.
So, really, the most pressing issue is speaking with your teacher and/or guidance counselor and, depending on why you missed it and school policy, trying to get them to either hold off on your grade until you take it in January, or give you an alternative assessment for your class grade and then take it in January to get the advanced diploma designation (if that is what you are going for).
Be honest and be polite.
If you messed up, we all do. Come clean, take responsibility, and explain what you will do to not make the same mistake again. Even if you were relying on someone else, you should still take ownership…even if a parent was supposed to get you up, it is still your exam, your responsibility.
If it was truly out of your control (you got sick, car accident on the way over, abducted by aliens), explain the situation and ask for their help and guidance how to proceed.
If you have other emotional/psychological/physical issues going on and do not feel comfortable sharing them, have your doctor or therapist or school counselor write a note or call to vouch for you and advocate on your behalf if necessary.
Good luck! It is just one test in one class!