My website is getting down after every Apppool recycle, do anyone have any idea how to resolve this issue.
Try to Disable the Overlapped Recycle of the application pool running your nopCommerce application. There was an issue with the shadow copies of the dll files of the plugins in 4.0. It should be resolved in 4.20, although I've seen it come up on a couple of our clients. Disabling the overlapped recycle should fix this issue for you.
Hope that helps!