Not sure what is going on as there isn't much detail about any discoloration or leaf symptoms, but you mention they are shriveling up, so I'm thinking it might be sclerotinia stem rot.
If your plants are very young, damping off could be the problem.
Any of that sound like what you are seeing?
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.