Computer was working fine then i decided to try a new motherboard in it cause i was bored, so i took apart my computer, put in the new motherboard, realized the cpu slot is different, then put everything back in and now 1 ram slot isin't being recognized by my computer, instead of booting with 8gb its booting with 4.

im not sure what to do anymore

Check if the RAM is properly inserted into their respective slots (reseating might be needed), and if you're not occupying all slots, check if they're placed correctly according to the motherboard manual.
And double check if the CPU power has been connected properly (example: http://i55.tinypic.com/2ecjhwi.jpg ).

Additionally you can try cross testing the RAM, by only running 1 dimm in the first slot on the motherboard, to see if you can boot the PC in that configuration (and to test the dimms individually).
If none of the above yields any results, and depending on what CPU you have, check if you accidentally bent any of the pins on either CPU or motherboard.

It would help greatly if you could list your PC's specs as well.

Or simply ESD fried it.