Those are roughly 7 year old systems. The first thing to do would be to purchase brand new CMOS batteries for them and try again. At 7 years the batteries could be weak at the very least. Then check all the connections in the previously working system. If the system is dead then it is very possible you disturbed something such as the main battery connection or some other component during the disassembly. Doublecheck all of your prior work.
-Forum Member since 2016 -I do not work for Dell -A user helping other users -If you like an answer please click on thumbs up -If a reply is a solution to your problem please click on Accept as Solution Gaming on a home built with GIGABYTE B560M DS3H AC budget build with an i3 10100 Real work on a Latitude E5430