It can be possible for many reasons.
If your domain account password was changed or not.
So set the properties of AX service as desribe below.
1) Right Click on your service and select properties window.
2) Go to the Logon Tab
3) Select This account on LogOn Tab
4) Enter your account in DomainName\Account
5) Then enter your Password and confirm password of account.
6) Now start the Dynamics AX service.