Check the XOUT and YOUT test points on the game board first, to make sure it didn't lose one axis. It may or may not be a monitor problem, so you want to confirm that first.
You should see between -1V and +1V DC for both X and Y, and roughly between 2V and 4V AC on both X and Y. (They will fluctuate as the game cycles through attract mode.) But if the Y AC is zero, it's a game board issue.
If it isn't the game board, start by visually inspecting the deflection board. Look for any cracked solder joints, or burned or damaged components. If there are any, just post them here, and I'll look at the schematics and try to triangulate what might have blown, to try to narrow it down.
If there are no visibly burned parts, test the 2N3716's and 2N3792's, as Robotronguy said. You can try testing them in-circuit, without removing them. Use the DMM's diode test, not a continuity test (google if needed). However if any are bad, they're more often a symptom of something else being wrong, and not the issue itself, so you'll want to test all of the other transistors for whichever axis it is, before replacing the bottlecap.
Also, there are two diodes, close to the large heatsink that the bottlecaps are on, which often short. Check those as well.