Right click the folder and select Add to mesh.
Or, go to the Live Desktop and select the sync properties for that folder. If it shows not synchronizing with the PC that seems to have lost its knowledge of being a Live Mesh folder, then change it to Sync with that PC. It should be re-offered to the PC and you can then choose to merge it with the existing folder. If it shows that it is synchronizing with that PC, make it not sync and save it, then change it to sync with that PC.
Before doing any of the above, confirm that you are signed into Live Mesh on that PC and connected.
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator