Firstly, make a backup of your project folder. (copy and paste it in a different location). Then you can open the backup and mess with that copy just so things don't get screwed up anymore in the original.
Then you can Delete the Perfect Parallel Folder, Copy the Perfect Parallel Folder from your known good course folder into your asset folder in the back up. That should bring the Perfect Parallel menu back. But your Terrain is still missing. If this all works and the PP menu is there, click the load terrain button and see what is there and it's location.
Check the Terrain settings in the inspector, click the cog wheel, make sure the Base Terrain "Draw" Box is checked. ( just above Pixel Error setting)
Your Terrain backups could of been saved somewhere else. If you named your backup, you could try doing a search in your computer.
Hope something works,, Jim
P.S. It is possible to reload the Terrain from your Lidar files without losing your Splines, but try these thing first. I can explain it later.
Gigabyte ga-z68x-ud3h-b3 Motherboard
Intel Core i5-2400 CPU@3.1GHz 3.70GHz
GeForce GTX 1050 Ti
16GB Ram (2)G.Skill Ripjaws F3-10666L9S-8GBXL
Windows 7 Home Premium