Hammer won't display grid
If you try to load or create a new level in the Valve Hammer Editor and the viewports don't show the standard grid, it's possible you just need to press Shift+R to toggle the grid, so go ahead and try that first.
Many new modders have been reporting that when they try to create new levels for their mod, Hammer will load, but it won't display the grid, and they can't add new entities to the map, and if it was a loaded map, they can't see anything except through the main camera. If you're having this problem, you probably also aren't able to launch your mod from Steam (even if you can launch it from your compiler). If this is the case, the problem probably lies in your mod's gameinfo file.
Check gameinfo.txt and check the number after SteamAppId. This needs to match the Steam Application ID of the base game you will be using to launch your mod. If the number points to a game that you don't have installed, you won't be able to launch your mod, and Hammer won't work either.