You can try to fix this yourself, but you might want to get a professional pinball repair person to do this work for you.
One of the sad things about pinball is that they break. They break a lot. Usually they break in easy to fix ways. Frequently they break in ways that require specialty or difficult to obtain parts to fix. Rarely, but often enough that you should take note, they break in a way that requires a technician to fix.
If you don't have a person who you can call to come and fix your pinball, I would suggest you get a person you can call to come and fix your pinball... sooner or later you are going to run into a problem that requires a technician to fix.
That having been said, the manual for your game is here:
https://www.ipdb.org/files/4505/Stern_2001_Monopoly_Manual.pdf
Optos are funny. If they are BLOCKED, they show as active (lit up squares) in the switch test on your pinball.
But blocked might also mean 'I can't see the light'. So... this should be visible... do you have red light shining down from the transmitter optos on the bank? The transmitter optos are inside the top of the bank assembly. They shine downward to the transparent windows where the optical reciever board is.
Do you have red light?
If not, you need to look at the connector to the transmitter board, and make sure you have +5v to turn on the LEDs.
The theory of operation for your 4-opto board is on page 130 of the manual (pdf page 147).
So, the transmitter LED's could be dark due to low power, or weak. LED's have a limited lifespan, and they get to 1/2 brightness after a certain number of hours. Because of this I've been replacing all the opto LED's with these:
https://www.mouser.com/ProductDetail/Lite-On/LTL2F3VEKNT?qs=Sv%252BigzQKaoVYLcEZH8BKGw%3D%3D
LTL2F3VEKNT is the part number.
They are very bright, and you would replace both the transmitter and the receiver LED's with the same part. Order a dozen. You'll need eight just for the bank optos if you suspect that the light just isn't bright enough anymore.
The receivers for the LED light beam have to see the light.
That means that the little transparent 'windows' through the playfield have to be really, really clean. I use q-tips and windex.
When you pull the Receiver board off the bottom of the game, clean the opto LEDs. I use Windex and a paper towel.
So far, we've covered what happens if you have no light, weak light, too dirty to see the light.
The receiver board has a connector. It needs those pins to have good wiring, and no cracked solder joints. Inspect the connector on the receiver board carefully, and reflow the solder if anything looks like cracked solder joints.
Finally,
I'm looking at page 16 (pdf page 33). The switch matrix chart shows that the common thing to all four optos for the bank is the green-black wire. The green black wire is also connected to the cop drop target switch, and the middle of the left ramp switch.
Make sure those switches are registering, and check carefully for broken wires. A broken wire at one of these switches might disconnect the receiver opto board... making it not able to tell the CPU that it was seeing the LED light beams.
That's where to start, let us know what you find!
