Without knowing the details of your situation...
You might want to start by checking to see if sql server agent is running and make sure it is using the right account.
If you discover no problems with sql server agent check the job history, check the sql server and windows server error logs.
Also, if your job ran successfully in the past, but no longer does so, make sure it is still enabled by examining the job properties in SSMS>SQL Server Agent.
Please post your findings.