Won't boot after new memory installation

MacBook Pro, mid 2010, 13” screen, MacOS Sierra. Max memory 2 x 4Gb.

Thought I had just one memory stick 2Gb, since that's what the machine said. Bought 2 x 4Gb, opened the Mac to change and found two sticks installed. Pulled them out, inserted the new ones, closed and tried starting. No go, just chiming.

After searching i.e. here and elsewhere and testing in various ways, cleaning both slots and sticks, I eventually found out that lower memory slot didn't work. Just chiming every time.

Then inserted just one stick in the upper slot, and the machine finally booted. I thought. Because the load bar stopped half way and refused to go further. Tried rebooting several times in different ways but no. Tried booting from a USB stick. Didn't work either. Loadbar stops half way all the time.

Suggestions to fix this would be appreciated. Or where to get a new "holder" for the sticks or some other spare parts that doesn't cost a fortune.

TIA for help. Cheers!

Danny