Have you checked that the cables are connected properly. Good to check the most obvious things first.
Does the drive show up in This PC/ My Computer? Does the disk title appear there?
When you insert disk does the drive spin up - can you hear anything?
Here is the URL for the “DELL Dimension E520 Service Manual”:
https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_dimension_desktops/dimension-e520_service%20manual_en-us.pdf
First run the DELL Diagnostics by restarting or turning on computer and immediately press the F12 key.
See page 22 of manual for DELL Diagnostics details.
You could try blowing dust off the laser - not while connected of course. Take the drive out of the computer.
Page 64 for DVD drive removal.
The other option is to simply replace the drive but do run the Diagnostics first.
If you need more help just ask.