System 76 Desktop questions...

I bought this one back in approx.2014, a wild-dog pro Desktop, w/ Ubuntu. I loved it, have had issues off and on, as I'm no Linux expert. I know just enough to get myself into a jam. Lol... Been messing with Windows computers since '98... but had a stroke in '08, and I'm not so swift anymore.

I have updated to the latest version a few months ago but was discouraged to find that I had no sound. (?)

Recently bought a new monitor for my desktop. When I hook it up and turn on the desktop power, it runs for maybe 3 or 4 seconds, then kicks off. And same thing again, and again. And nothing shows on the monitor. Light underneath shows it's on, but no picture. It did that last time I had it up and running, and it straightened out but I can't remember what I did to get it working last time. Maybe something isn’t hooked up…? The desktop didn’t come with much for instructions, if any.
Suggestions?