If something goes wrong with SharePoint Search 2013 and under Search Application Topology section if you see Search Index partition error – a yellow triangle. This defenitely meant something is wrong with Index partition in that particular server.
Try the following things before you pull your hair, or reset index or even rebuild the whole search service.
1. Clear the SharePoint Server Cache. I listed the details of this process in another blog here.
You can also do this using Powershell
net Stop OSearch15
net Start OSearch15
This should be performed on all the servers in the farm which are running Search service.
If Indices are not corrupted and nothing serious has happened to your search, after clearing cache and restarting the service, your Topology should show up just fine with tick marks under all the components. You can do a full crawl and get search back on track.
This worked for me and may not be the correct procedure. Hope it helps someone.