Go outside and look for a manhole cover over an inspection pit - if you find one peek inside - if it's clear then the toilet is blocked - if its partially full (it should be empty) you will know what to do next. Either yourself - a very good friend or a drain professional.
You can clear a blockage either by pushing (may make things worse) or by pulling (much safer).
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.