dear friend , thanx for sharing you concern,
don't worry, your dentist
must have done RCT treatment for your abscess tooth, but i think you didn't place any prosthetic crown on it . so the tooth got brittle and broke off .
now you don't have to worry at all, again this can be done.
regarding your sunken cheek, definitely when crown will be placed after treating the tooth your dental appearance will change and will improve .
the cause for your cheek swelling was the periapical abscess
which will disappear after getting proper treatment done .
i would recommend you to do the following ;
1 seek your dentist help.
2 get an xray done , understand the underlying cause for swelling .
3 if required get started with antibiotic treatment followed by RCT .
4 get prosthetic crown , made for that.
this will solve your problem.