Exchange Information Store on server is inaccessible

One of our users is unable to send emails. They recieve ones sent from other users but when they send one, it is not received. The emails appear in their sent items. 

I logged in to manage Exchange server, double click on the user account, then went to Mailbox Usage and get the following error - 

Exchange Information Store on server 'amspr05mb262.eurprd05.prod.outlook.com' is inaccessible. Make sure that the network is connected and that the Exchange Information Store is running.

How can we fix this users account?

Thanks,

M