If you use flash_recovery_area, increase the size there:
$ sqlplus /nolog
SQL> connect sys as sysdba;
Enter password:
Connected.
SQL> select * from v$recovery_file_dest;
if SPACE_USED is the same or close to SPACE_LIMIT it's definitely this reason (no place to store archive logs)
increase the size of flash_recovery area:
SQL> alter system set db_recovery_file_dest_size = 50g scope=BOTH;
this will set the size of the flash_recovery_area to 50g immediately, and after restart as well (setting the parameter to SPFILE)
If your database is not opened, try to shut it down first.
SQL> shutdown immediate
if that takes long time then
SQL> shutdown abort
SQL> startup nomount
do the stuff written above
SQL> alter database open;