I was earning about 3.00 dollars a day with adsense on one of my sites. And because of some difficulties with my host the site was down for a couple of days. Now my earnings from adsense on that site are 0. Does anyone know what could have happened.

probably purely coincidental

Or it could be that adsense is not displaying ads but PSA's.

Adsense directly depends on traffic. Is your traffic the same after down? One of my sites was being crawled when the served went down so after it was back up it lost 60% of the total traffic which means 300 USD less every day. I was really upset by that fact.