We use because, since and because of to give a reason why something has happened. But we use sentence after since & because. After because of, a gerund or a noun is used.

 

Because       + Sentence                                

***   He had to leave the job because he had some problems with his boss.

 

Since           + Sentence                                

***   He had to leave the job since he had some problems with his boss.

 

Because of + V_ing / noun / noun +adj          

***   Because of some problems with his boss, he had to leave the job.