What have you tried? Have you you tried using another monitor? Are you sure the cable works? Does anything display on the screen? Can you get into the BIOS? Press F2 key just after the Dell logo appears when turning the power on.
Any lights?
Click on "Service Manual" on the line below for the Dell OptiPlex 160 Service Manual
[link|https://dl.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-160_service%20manual_en-us.pdf|Service Manual|new_window=true]
Look in the Troubleshooting section for the power light codes and see if you have a match for your system.