Sounds like a possible MPU issue. Since you are new. Prob a good idea to read through this first. http://pinwiki.com/wiki/index.php?title=Williams_System_9_-_11

Did the game work before you bought it? Or did you know it was broken. Do you know how to open the back box? If so hows those batteries looking? Corroded or leaking acid?

Was it on free play? Did the display work before? If so. I would get a Multi Meter and test all you voltages on the power supply first. Its important to make sure you dont have power issues

When you get a chance Take a photo of the batteries in the back box. And like I said we should start with easy stuff. Kinda take the temperature of the machine (figuratively) check all the Test points for voltage test fuses ECT. Sounds like maybe the power supply shit out as a preliminary guess but it could be a whole his of issues

Well that #7 code means this. System failure. Occasionally, the following components can contribute to this problem: U21 (4MhZ crystal), components in the IRQ circuit, broken leads or failed C9 (22uF) in the reset section, or loosely seated ICs on the CPU board. Note that boot code "7" is the hardware default and characteristic of the CPU "reset" signal not complying with the low-to-high spec properly at pin 40.

1) Read through this thread. https://pinside.com/pinball/forum/topic/space-shuttle-wont-boot-error-code-7-williams-system-9

2) Do you have a multimeter?

if not

3) Maybe see if the Main IC chip came loose try taking it out and carefully and put it back in. WITH game off. use small flat heat screw driver gently lift each side.

if so

3) Do some preliminary work,

A) like test all your fuses (out of circuit and 1 at a time so you don't confuse them)

B) On your power supply board there is a number of test points, get the voltages readings on each one. Lets makes sure your not dropping voltage. Could be you lost some power and the CPU isnt running causing your errors.

C) Maybe see if the Main IC chip came loose try taking it out and carefully and put it back in. WITH game off. use small flat heat screw driver gently lift each side.