Presumably, its te downstairs wc that is blocked, and the upstairs are still working? If this is the case, and they all connect to the same pipe, then probably there is a blockage underground. when an underground piipe fractures or tree roots work there way in, then 'larger objects' such as wads of toilet paper will get snagged, and other stuff will also get caught and it all backs up. Liquids/water can seep past and lower the level for a while, but eventually it will back up and fill up the manhole, and then back up the piipe, so the wc can't flush. If this i the case, then either buy some drain rods from Screwfix, or call out Dynorod or similar. If you use drain rods, with the 'swirly' bit on the ends, ONLY TURN THE RODS CLOCKWISE when in the drain, otherwise you'll unscrew the end, and it will fall off, down in the drain, and add to your problems.
So step 1, find manhole, lift lid, if full, go to plan B, if not, and there are multiple branches, get someone to flush toilets in turn to see what happens. If all but one work, then rod backward to the wc from the manhole.
Can't be any more help, sorry.