However, a lot of these machines have a last contact time of that day or some other time for any given week. I have tried to do an uninstall and then an install as this has worked in the past to reinstall the agent on a PC that I know is talking to the server. 

My question to the community is this. How would one go about getting those machines with the downed agents to come back up again and talk to the server, or to even find out if they're used anymore? Any help would be appreciated, as I can't go around and try to hunt down these PC's one by one.