If you don't see a backup.mbz file in the directory where the backup was being built then it didn't finish. A clue is presence of a moodle_backup.xml file. if that is not there, it did not finish.
Not finishing is an indication of php time outs OR lack of resources of server OR restrictions.
Check backup preferences.
If you see the default - files area - that's the sea of files in moodledata/filedir/ which are not humanly recognizable.
moodledata
filedir/
has a bunch of subdirectories named with 2 character/number names.
Looks like:
xx yy z7 8g - a bunch
inside xx there are subdirectories that are of the same - 2 character/number names
and inside of those are files that have the contenhash value:
look like: xx/8g/xx8gbunchoflettersnumbers
one of those xx/8g/xx8gbunchoflettersnumbers is the backup file.
Use any tool you have to browse files and navigate to your moodledata/filedir directory. You'll see what I have tried to described above.
*IF your backup preferences are set to use a 'designated directory', that's one you create outside of moodledata/filedir/ and you will be able to see humanly recognizable names.
You have to create that designated directory AND moodle must be able to write to that directory (ownerships/permission).
'SoS', Ken