If the HDD is dead and the power-on test is turned off, the board will appear completely dead save for some LED activity as the watchdog resets the power-up sequence. This is very common for "dead" Seattle boards that show up on ebay. Turn on power on tests (I forget which DIP SW, but it's the same for all versions of Blitz) and see if it'll at least come up and attempt to spin up the HDD and load a program. If it gets through the POST but resets (it's a watchdog reset) after sitting at a blank screen for a while, your hard drive is bad. If it stalls out on the Hard Drive check, then clearly the HDD is bad.
If the HDD is bad, you won't be able to run the test mode program as that gets loaded from the disk drive, so enabling power-up tests is the best first course of action on a non-booting Seattle.
Hard drive failure is by far the most common problem. They are pretty old at this point, and the commonly used Quantum Fireball wasn't the most reliable thing to begin with. Also make sure you have very good power. If necessary, power the HDD directly from the power supply using dedicated wiring rather than running off the PCB's connector which draws from JAMMA. Those HDDs are very picky and will spin up and down constantly when given droopy power usually resulting in either bootup problems of missing graphics during gameplay.