hi - sorry to hear about your situation!
here is the page from gmac's official site describing score cancellation procedures.
gmac's policy states very clearly that they don't allow cancellation of scores, for any reason, after the scores have been viewed, so your odds are not very good.
a couple of things:
(1) first of all, and most importantly,
you shouldn't worry too much about the outcome of this exam. remember that
schools will look only at your best score, so, as soon as you've retaken the test and received a better score, you can forget all about this fiasco.
so keep your head up, forget about it, and learn your lesson about better judgment (see point #2, re: better judgment).
in the very, very unlikely event that a school asks about your 'disastrous' score, you could produce the hospital documents you've mentioned, by way of explanation.
(2) there is a very strong lesson to be learned here, and that is the following: tenacity is admirable to a point, but excessive persistence is foolish.
in other words, you need to realize that it's sometimes better to give up!
you said:
I suffered serious dental pain despite taking a couple of super strong pain killer, especially in the Verbal section, the pain become so bad I couldn't really think.
but then you said:
I didn't realise how serious the problem was
i actually find this combination of statements a bit worrisome: you had dental pain so bad as to interfere with your mental processes - and force you to pop painkillers during the exam - and you
still didn't realize that your performance was compromised.
i'm worried for two reasons:
1, generally - i'm afraid that you are so hard-driving that you would sacrifice your own health to avoid backing down even a little bit, and
2, for the gmat -
you should concentrate on getting your mind and body in a proper state for the test. the flipside here is that, if you don't realize that excruciating pain and pill-popping is a
bad physiological state for the exam, then you probably haven't thought enough about what is a
good physiological state for the exam, either. so, before your next gmat administration, put some serious time and effort into thinking about an
optimal state of mind and body, and do whatever is in your power (including diet, sleep, and REST) to attain that optimal state.
(3) go back and read #1 again - your messed-up score doesn't matter, as you can always re-take the test.
if, after reading #1 again, you still want to
try contacting gmac, then write a formal letter, make copies of all your hospital documentation, and send it to the address indicated on gmac's website.
if you do this, go ahead and update us when you hear back. we at mgmat would be astonished if they are willing to cancel your scores, but i suppose you can't find out unless you try.
good luck - and try to work on your self-awareness!