Improperly flashing firmware can "brick" your router, rendering it completely useless. Follow these precautionary steps before starting the update:
The router will automatically reboot itself once the upload finishes. The LED lights on the physical device will flash, turn off, and slowly power back on to a solid green or blue state. D-link Dsl-124 Firmware Update Download --BEST
You have a "soft brick". Unplug the power cord for 10 seconds, then plug it back in. If the router works, try the update again from a different browser (Chrome often works best; avoid Safari or Edge legacy). avoid Safari or Edge legacy).