Question Installed ram and now the computer won’t display any thing and beeps 3 times (long beeps)

May 1, 2019
Cpu i5 8400
Gpu 1050 ti
Motherboard Msi h310m pro-vd

I already have a Corsair vengeance lasts lpx 8gb stick installed 2400 MHz then I tried to install a crucial Ballistix 4gb DDR4-3200 udimm then when I booted up the computer the pc didn’t recognise the ram even in the bios then I swapped the sticks around the two slots and the computer won’t output a signal but it just beeps 3 long beeps and stops.
That mobo only supports 2666 speed RAm so not sure why you spent the money on 3200.

But to answer your original question the reason the new RAM may not work is because its either defective(which I doubt) or its simply not compatible with your mobo. As far as running them together the 3 beeps mean bad memory, which again may not mean defective but simply it wont run that particular stick.

Here is the RAM QVL list for your mobo Click the Memory by i5/i7/i9 tab for the list.
The other thing to consider is that mixed RAM modules are not guaranteed to work together, irrespective of if they are the same speed / frequency / make / model / size.

RAM modules are sold in the package sold because the modules in one pack are guaranteed to work together.

Mixed RAM modules is a very mixed battle, sometimes it works, sometimes it doesn't.