To restore the use of your function keys...
Go to System Preferences > Keyboard > Short cuts tab > Reference the pic below.
On the left-hand side, click each one, then click "Restore Defaults" on the right-hand side.
Not all of the shortcuts will have a 'restore defaults' option. For those, just ignore it and move on to the next.
If your screenshots are not being saved on the desktop, do the following...
- On the top-right of your screen, click the magnifying glass and type 'terminal' in the Spotlight search field, and press enter.
- A text window will open up.
- Copy-n-paste the following and press enter.
defaults write com.apple.screencapture location ~/Desktop
- Try the screenshot option again.
- type exit in the terminal window and close it.