At what point in history did Tzra'as stop happening? Was it sudden or gradual? Do we have any sources that suggest that it could still occur today?
If nowadays a Cohen declares somebody (or a garment or a house) as having Tzara'at, then that declaration is valid and the laws of Tzara'at kick in. All this assuming that the "blotch" in question has the correct size and color.
The Ramabm in הלכות טומאת צרעת פרק י"א:י"א - says that the process for "curing" the Mezora from Tzara'as is followed even nowadays - even outside of Israel.
Note that the last step in the "curing" process - that of bringing a Korban - is not done, since we don't have a Bet HaMikdash - and is not needed unless the person wants to eat Korbanot (which can't be done nowadays anyway.)