Outlook 365 no longer receiving/sending emails

I have office 365 and windows 7 on my laptop. I use outlook to access work emails and it was working perfectly for about a week after I installed it. Now it won't sync, receive or send emails. All the subfolders are there but there are no emails. I have done the subscribed folders thing, and the root folders thing, even modified the registry key as advised by microsoft and set outlook to default mail client but absolutely none of these has cured the problem. what on earth can I try next? Microsoft have been absolutely no help at all as usual!