Don't panic. Start here.

I just clicked the wrong thing.

Press ⌘+Z immediately. This undoes typing, file moves, deletions, almost everything. Press it more than once to keep going back.

I can't find the file I just saved.

1. Press ⌘+Space and type the file's name.
2. If you don't remember the name, open Finder → click Recents.
3. If you downloaded it, check Finder → Downloads.

An app stopped responding.

1. Press ⌘+⌥+Esc.
2. Click the stuck app from the list.
3. Click Force Quit.

The other apps are unaffected.

I closed a browser tab I needed.

Press ⌘+Shift+T. Press it again to keep reopening older tabs.

I deleted a file by accident.

1. Open Trash (bottom-right of Dock).
2. Find the file. Right-click → Put Back.
3. It returns to where it was deleted from.

The cursor disappeared / mouse not responding.

1. Wiggle the trackpad with your finger, Mac will magnify the cursor.
2. If still nothing, close the lid for 5 seconds, open it again.
3. Last resort: hold the power button until the Mac shuts down. Wait. Power on.

The screen went dark.

1. Tap any key, or click the trackpad.
2. If still dark, press the power button once.
3. Check the brightness key (top row, sun icons).

Typing isn't going where I want.

Click once on the box or window where you want to type. Mac follows the click, whatever you clicked last is "active".

An app window disappeared.

1. Look at the Dock, does the app icon have a small dot under it? If yes, click it once.
2. Press ⌘+Tab and pick the app.
3. Or press F3 (or Mission Control key) to see all open windows.