Hi, everyone

I need your help to understand and fix this problem:

I built an apex text class in a sandbox environment (i'm using eclipse) but when I run this apex test class I'm getting this error: System.QueryException: List has no rows for assignment to SObject. The problem is that the object really has records both in sandbox and in production environment !!!! I run that query in Force.com Explorer, salesforce.shema Explorer, etc. and there I can see the existing records but in the test all the time throws that error... Could anyone please guide me why is it happening and how can it be fixed?