I must have accidentally activated some keyboard shortcut function because every time I press certain keys such as w, a, s, d they act as shortcuts to things. For example if I press f, the search function appears. How can I stop this so I can type these letters again?

Thanks

In System Settings > Keyboard > Shortcuts

Find the shortcut you're trying to disable, click on it to set a new shortcut, then pressing Backspace will disable that shortcut.