I get the message, no video input. Trying to connect my new acer pc with old ph

Make sure the graphics card is well seated on the mobo and that the power cable(s) that go to it are not dislodged (could happen in transit). Do this with the PC powered off and unplugged. After that, ensure the monitor cable is in good condition, no bent pins, then hook it up to the PC again and reboot. make sure the monitor is also plugged to power.