Clear your outbox and try again. Sometimes an email can get stuck and will cause further emails to not send. Another thing to check would be to sign out and then sign back in to your account in case you have been timed out or information has changed. Make sure that work offline is not selected as well. Here are some steps that Microsoft recommends if your email client is Microsoft Outlook (a similar process can work with other email clients): http://support.microsoft.com/en-us/help/2663435/email-stuck-in-the-outbox-in-microsoft-outlook. If these type of troubleshooting steps do not resolve the problem, check with your email service provider for further assistance.