Help! Email could not be sent?

I have just imported almost 4000 members from Ning into my new Social theme site,

When I tried logging in with my password from the previous site it didn't work. I didn't get the email that was supposed to let members know we imported them.

When I tried resetting my password, this is the error I got:

The e-mail could not be sent.
Possible reason: your host may have disabled the mail() function...

How can I fix this? I am on a single WP install.

Thank you!