WHY CAN'T I SEND ANY EMAILS IN WEBMAIL??

I have used TalkTalk Webmail for years and it continues to work fine on my iphone through the MyMail app, but since a few weeks ago, when I log into webmail on my PC I am unable to send any messages - A 'Failed To Send' banner appears every time.

I have avoided needing to use it, and hoped it would sort itself out, but it hasn't and now I need to send an attachment so need to use the PC not my phone.

Any ideas what's gone wrong/How to make it work??

Thanks