i pulled out old ram and plugged new ram into pc. Now it doesn't register and won't connect to monitor. what is happening, just blank screen?

RAM has a SHAPE if you like. think of a chessboard representing the space available. then think of a RAM as occupying squares. so a 2x4 ram occupies the same space as a 1x8 but they have a different shape. you may be trying to fit a 2x4 into a 1x8 slot or something like that. you must also obey the other rules such as the CL value(cas latency) must be right, and some memory only works in pairs. paired slots are usually 0,2 and 1,3 so are not usually next to one another. hope that helps

I pulled out my CPU and I can't figure out why my PC doesn't work. I put it in a trash compactor hoping this would fix it, but no luck.

new ram not working is first guess. check mobo support information for correct size and speed

Wiggle the RAM sticks a bit in place then try starting again.

If that didn't work, take them all out, get a new pencil with an unused eraser. Use the eraser to clean the gold contacts on the RAM. Be careful not to scratch any other part especially those tiny chips very close to the edge. Reinstall one at a time, starting the computer each time. If the computer fails to start, the last RAM stick you installed my be damaged.

Try to reseat the memory/ram in the motherboard

Or try to use the memory/ram in a different DIMM slot on the motherboard.