My car will not start. Won't turn over. No lights come on. Nothing. (1977 Corvette)

Hi,

My car will not start at all.  The battery was taken out and taken to two
different places, to check it.  Both places said the battery was fine. I bought
this Interstate battery in 2017.

The car is in my driveway.  The shop I go to, is literally 1/2 a mile from me,
one road over.  Under a minute away.  I could have the car towed but it
would be between $65.00 and $125.00.  I called places today.  I would rather
have a mobile mechanic come (this time) to fix it.  

When you try to start the car, nothing happens.  Not a sound.  No lights
come on, no needles move in the dash.  Total silence.

In 2015, I had a starter put in, and a clutch safety switch put in, as well.  
Some people are telling me, they think it's a ground strap that has come
lose.  I have no idea what that is.

I know I have to have someone look at it, but just from what I have written,
could anyone tell me what it sounds like to them?  I know nobody will know
for sure, until it's seen, but I would appreciate any feedback.  

Thank you.

Beth