I got the following error when my MacBook abruptly restarted:

Bootstrap failed: 5: Input/output error
Error: Failure while executing; `/bin/launchctl bootstrap gui/503 
/Users/mwal/Library/LaunchAgents/homebrew.mxcl.postgresql@11.plist` 
exited with 5.

Here's what I tried to resolve the error:

After checking the ports and trying out all the possible solutions, I decided to look into the logs.

tail -n 100 /usr/local/var/log/postgresql@11.log

This gave me the last 100 lines of the log where I found this:

FATAL:  lock file "postmaster.pid" already exists
HINT:  Is another postmaster (PID 479) running in data directory "/usr/local/var/postgresql@11"?

Clearly, the issue was with postmaster.pid so I deleted it:

rm /usr/local/var/postgresql@11/postmaster.pid

Started the postgres service again:

brew services start postgresql@11

and voila! It worked.