I'm making stewed tomatoes & forgot to put the lemon juice in before sealing the jars. Will they still be ok or should i start over?
Unfortunately you should start over. The lemon juice (or citric acid) is necessary to get them acidic enough to prevent botulism growth, since tomatoes aren't quite acidic enough on their own.
That means completely starting over, in particular clean and re-sterilize the jars.
You can reuse the lids, though. It does mean slightly increased risk of them not sealing quite properly, but it's still pretty unlikely, and you can just test the seals to catch it and keep whatever doesn't seal in the fridge or freezer.