all the advice above is great…something to add if you dont have and ab wheel…you can just take a barbell, load it with 45’s, and do roll outs with it, better if you have bumper plates…you can load the bar up for more resistance on the negative since you will have to control the weight on way down and have more resistance on way back up.
Also, its not direct ab work but heavy squats, deads, cleans, overhead squats, snatches, and doing ground based overhead press, push press, etc will strengthen your core in a very functional manner! Forget using a belt! If you can squat/dead/clean/overhead press/overhead squat/snatch heavy you will get strong abs and lower back.
Med ball routines are nice too, all kinds of things you can do with them, lots of throws, rotational work, if you need ideas ask and ill try to post some med ball stuff but many books and im sure articles on the internet have lists of exercises. “Iso” abs also will hit them hard, basically get up on your elbows in push up position and hold a tight flat back for a while, then turn to the side and do it on each elbow as well. I dont know what others call it but we call it “alligator walk” which will hit your core and shoulders,get in push up position with a 45 pound plate at your toes and drag it keeping your body tight using your arms to pull you, might seem like arm work but it also forces you to keep abs tight to stabilize…
hope this helps…but yea you gotta heal up before you do anything. Make sure it isnt so bad you need to get surgery first.