It is actually quite difficult to accidentally bulk delete assets so the first thing is to check that the assets have actually been deleted. It is more likely that you have removed them from a category, i.e. the assets are still in the library but they are not showing up in a category they used to be in.
You can see assets that are not in any category ('orphaned assets') by going to the 'advanced search' page and checking the checkbox at the bottom labelled 'find only items that are not in any category'. If you want to add all of the resulting assets into a category then you can do that by repeating this search on the 'bulk update' page.
The only way to really bulk delete assets is using the 'export' functionality that is available to admin users only. This involves clicking the link 'export results' at the bottom of a search results page and then checking the checkbox 'delete assets' on the last page of the export process.
If you have deleted assets that you did not mean to delete then the only way to get them back is to restore your library from the backup files (backups should be performed nightly) - contact the technical contact who looks after your server.