First post here, but I've experienced this before. No impressions for a whole day no matter what I did (pause, restart, raise bids, new ad copies, etc.). I thought it might be a billing issue (normally they charge my accnt. every evening, the morning following the one evening they DIDN'T charge me, I got 0 imp all day). I contacted my rep, he told me they couldn't view any of my campaigns, he spoke to their "engineering" division, they never figured it out.
This was about a month ago. I tried copying ads into new campaigns that day but eventually just gave up and sat it out. Not a big deal and a nice break, things resumed normally the next day.
This also happened to me today, in my two main campaigns. This time ads copied into other campaigns are running as usual while those in my two main campaigns still have 0 imp. Don't sweat it too much. Your ads will probably resume tomorrow as if nothing had happened.
Good luck.