No Power

The most common cure for a DL4 without power is to remove the back panel, and then lift out the EPROM (U11), and then put it back in. That's right: just in and out.

After this, do a normal reset by powering on while holding down switches "A" and "TAP TEMPO."

On most units, this will do it.

On some units, the power supply may have been damaged. There are several ICs and diodes that may need replacing. Replace them all. Some are fairly expensive.