Is the agents still connected to the collector? Is that collector still connected to the appmon server? If not, i'd look into possible firewall issues between the components. That's the easiest thing to overlook.
I'd also check that you don't have any filters in place that may be looking for the old infrastructure. A screenshot of the entire dashboard would probably help in addition to the agents overview.
If they actually are still connected and to the same agent groups the question you want to be answering is if the measures used in the charts are still getting data - you can chart them on a new dashboard to see.