Is the server running on host "localhost" (::1) and accepting TCP/IP connections on port 5432?

run postgres -D /usr/local/var/postgres and you should see something like:

 FATAL:  lock file "postmaster.pid" already exists
 HINT:   Is another postmaster (PID 379) running in data directory "/usr/local/var/postgres"?

Then run kill -9 PID in HINT

And you should be good to go.

You most likely ran out of battery and your postgresql server didn't shutdown correctly.

The easiest workaround is to download the official postgresql app and launch it: it will force the server to start (http://postgresapp.com/)

Most likely it's because your system shutdown unexpectedly

Try

postgres -D /usr/local/var/postgres

You might see

FATAL:  lock file "postmaster.pid" already exists
HINT:  Is another postmaster (PID 449) running in data directory "/usr/local/var/postgres"?

Then try

kill -9 PID

example

kill -9 419

And it should start postgres normally