If you are on Server/DC, you need to talk to your administrators.
If it is Cloud, then Atlassian are in charge. They are not showing any incidents at the moment, ( see https://status.atlassian.com ) so you will need to report this to them at https://support.atlassian.com/contact
You'll also need to clearly define "can't use Jira" - is it offline? Or you can't log in? What are you getting? Error messages?