I've pitched my yeast and nothing is happening.
More often than not you are experiencing a bout of delusional paranoia. No really, things usually end up turning out just fine for most worried brewers.
You may also be experiencing an extended lag time due to under-pitching yeast or insufficient aeration of the wort. If you really believe that nothing has happened, we urge you to take a hydrometer reading before taking it to the next panic level.
If for some reason you were born under a bad sign and nothing actually did happen you can re-pitch a sufficient quantity of yeast. For this reason it is ALWAYS a good idea to have a few packets of dried ale and/or lager yeast on hand in the extremely rare event that you should need it.