I’ve installed a new graphics onto my Optiplex 755 recently and it was working fine; but today it both screens randomly says ‘no signal’ and the screen goes black. Any suggestions ?
If it worked fine before, it will be compatible, it wouldn't work at all if it wasn't compatible.
What is your full system spec including PSU make and model?
Have you tried a different display cable? Are you using any adapters?
Had anything changed before they stopped working?