By mistake, we have deleted our feature layer services. I want to restore these to our portal again. Can anyone help into this how I can get those services again?
Unfortunately, we do not keep backups of any hosted feature layer data for ArcGIS Online, You will likely need to re-publish your services from any data you have saved on your computer.
To piggyback off of the above comment, there's also an option in ArcGIS Online Item Details called "Delete Protection". If you enable this setting on a service, the service won't accidentally be deleted. Even if you try to delete an item that has Delete Protection enabled, the delete will process, however the item will still remain in your content.
Here is an article outlining this: Protecting your ArcGIS Online items.
Best of luck,
Retrieving data ...