The traffic API was returning data fine yesterday but now it seems to be not returning any data.
And it doesn't seem to be just me, take a look at this little JS app (isn't mine) http://jsfiddle.net/chufol/78mJV/show/ which was working fine yesterday (it was showing lots of road works etc), but now it shows nothing. (Info about that JS app http://www.mandogroup.com/blog/2014/displaying-live-uk-traffic-incidents-using-bing-traffic-api/)
It looks like your account is over 6 months old.
They are also rate limited when there is a lot of load on the servers or when your account makes a high frequency of requests. This is done to give priority to enterprise accounts.