MongoDB Failing to Start - ***aborting after fassert() failure

The problem can also be due to mongodb using up all the disk space. That was the case for me. You can install ncdu with this command: sudo apt install ncdu

This will help you in looking at the disk usage. Use the command: ncdu -x /

Remove the unwanted dbs from /var/lib/mongodb/

SOLUTION by kometen

Does the file /tmp/mongodb-27017.sock exist? If so try to delete it, ie. 'sudo rm /tmp/mongodb-27017.sock'.

sudo mongod

(suggested by @João Bortolosso)

after (suggested by @fresz):

sudo rm /tmp/mongodb-27017.sock

worked on mac!

If you use Ubuntu, try:

sudo mongod