Can I add frozen vegetables to my thawed chicken meat in a slow cooker?
Most slow cooker instruction manuals tell you to put the food in hot for most recipes. For traditional crockery inner pots the list of foods that can be cooked from cold (never mind frozen) is quite short and consists mainly of foods that are stable at room temperature. So you were marginal to start with (less so for some "auto"  modes). 

If you put the veg in then realised (almost) straight away, just bring it to the boil and carry on cooking. Of course don't use a crockery pot on the stove top or a metal one in the microwave. 

If it's been sitting lukewarm for some time you're on your own. It couldn't be deemed safe.