You should start by looking in the dba_recyclebin and see if your tables are there. You can also look at your archive log files directory to se if a lot of archive logs were generated around the rime someone else was changing your character set.

I doubt that changing the character set would drop tables, but I suppose it could happen depending on the name of the tables in question and assuming that you went to a more restrictive character set and the names of those tables could not be directly represented by the new character set.