If you find you cannot send or receive emails then first check you are connected to the internet. E-mails are sent/receive using the internet connection so if you do not have internet then there is no way emails are going to work.
There are several things which could be affecting your email service:
1. There's a problem with your email account
If you're not using it already try accessing your email service online via the Microsoft Office 365 portal and send yourself some test messages. If these all work the issue may be with your email software or device.
2. A large email is being sent
Large email attachments can cause your mailbox to lock, but it will unlock itself within 20 minutes. If the problem keeps happening, try deleting any emails over 3MB in your Outbox.
3. Your email account is full
Log in to your Office 365 account and try deleting any emails you don't need - particularly those with large attachments.
4. An issue with your email software or device
If you can log into your account, but can't send or receive emails, your email server settings may have been changed, so you'll need to check these. Contact our support team to go through these here.
5. Incoming emails are being identified as spam
If you're having problems receiving some emails, they could wrongly be being tagged as spam. The sender will get an error code 550. See further details on stopping this problem here.