Quoted from PinMonk:grab it and pull it back, or put a flathead screwdriver shaft on that post to the left and on the left side of the target and use that for leverage to force it back over.
There is a modified version of the target bracket you can install that prevents leaning. Pinball Life has it:
https://www.pinballlife.com/mm5/merchant.mvc?Screen=PROD&Product_Code=BK3
Ive done this more than once. Even tried locktite. It always goes back tonthe left. I want to order a bracket but shipping to Canada was like $40 for 1 little bracket