There are four case versions of the Dell Optiplex 760.
You are going to have to do some work and perhaps a lot of work to figure it all out.
FYI:
https://dl.dell.com/manuals/all-pro...desktop/optiplex-760_service manual_en-us.pdf
For example: the Desktop version maintenance etc. begins on Page 55 of the Service Manual.
Use the Service Manual as a Guide and Reference to determine exactly what may be in the build and what may be missing.
If not the Desktop version, then start with appropriate page for whichever case version you have.
Also go to Dell's website (be sure that it is Dell's website) and look for the applicable User Guide/Manual matching the pc.
If possible, take some photographs. Front, back, and inside.
Especially the motherboard.
Post the photographs here via imgur (
www.imgur.com).
The scope of the problem(s) needs to be narrowed down.
Unfortunately, the pc may be beyond any repairs and restoration to a fully working computer.
And you do not want to spend any money just to learn that.