For the last few months, this has been happening to me and its really annoying. If I update npm without root privileges like this
$ npm update -g npm
while I know I should have done this
$ sudo npm update -g npm
updating npm fails with
EACCES & the symlink from
/usr/local/bin gets deleted.
If you run npm now
$ npm -bash: npm: command not found
To fix this, just recreate the symlink
$ sudo ln -s /usr/local/lib/node_modules/npm/bin/npm-cli.js /usr/local/bin/npm
and npm is your friend again.