Exercise Question

I work out regularly. However, the last month has been non existent due to nausea and lack of energy. I'm ready to get back in the gym to get myself moving again. My question is, is there a safe heart rate zone I need to keep myself in? I'm on the heavy side already and have no intention of going in there thinking I can hit the elipictal full force. I just want to get myself back in the groove of things the safe way. Any tips would be grateful.