1) Check application log, probably you will see the reason of failure there

2) Do you remember the location of system database files(mdf and ldf)? Check if the files are available there or not.

3) Go to run-services and check the account configured for sql services? Is the configured account exist? The other possibility could be of password is changed for that configured account.

4) You can try to run the services on different account which have enough permission and you aware of its password.