If the drive is connected via USB, pull the plug and reinsert. This works for my Pioneer drive.
How did you eject the DVD? By a button on the front of the drive, or via the menu bar on the computer?
iMac (19,1, 3.1 GHz i5, 10.14.6, 40 Gb RAM) with Mac Pro (5,1, dual-2.4 GHz, 10.13.6, 24 Gb RAM) used as a server and to run 32-bit apps via Screen Sharing. MacBook Air (1.8 Ghz, 8 Gb RAM, 10.14.6, 256 Gb SSD) all patched through a Vodafone router and BT Wi-Fi Extender, Canon TS8351 printer/scanner.