Problem with threads not catching

Question

I am new to embroidering and I have just recieved a used deco bernette 600 from my mother-in-law. The machine worked well for the first 2 or 3 things that I embroidered. Currently, I am having difficulty with the machine. I get an error message saying Check upper or lower threads may be cut . I have looked it all over and what it is looking like to me is that the upper thread is not catching the bobbin thread. I have tried changing the bobbin the tread and the fabric. Can anyone tell me what could possibly be the problem and how to fix it ? Thanks