User-registration password notifications not sending.

I think that most of the people that have registered on our website got their email with their password at first but I dont think its sending emails anymore. I’m not sure on how to solve this issue and im not a super tech person. I made a test account to see if I would get the email or not and I did this but I never received an email (though my account has been established on our server) then if I try to reset my password (for test account) this message appears,

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

If anyone has any ideas or solution I would really appreciate it. I tried reading a lot of other topics but I never really found one with a solution. My hosting service is one.com