If you find you cannot teleport to a region and you know it was previously working, which means you got the port forwarding set up correctly, and the ip addresses are all correct, then go to that console for that region, and type: fcache clear.
If you run opensim on windows, you can use a program suchas "restart on crash" to monitor and restart the Robust.exe should it either crash, or start not responding. Restart on Crash is a free windows application.
If you are unable to stop a region from restarting, observe the logs. I went to visit a grid and got items, unfortunately while I was there the region crashed. I logged back in to my grid and rezzed the items I got previously from the sim that crashed while I was there. The next time I restarted my grid, that region restarted over and over. I found that the items had not finished downloading, and were trying to contact the grid that crashed. After I deleted the objects from the console using: delete object id "whatever uuid number" everything returned to normal and ran smoothly.