If you wrote a check that has not yet cleared, you may want to stop payment. If you wired money, or mailed a money order, you may be out of luck. There are lots of rental scams out there. Never send money for rental deposits unless you have seen the property and have met with the owner or property manager. And, if the deal seems to good to be true, it's probably a scam.
I hope you were not a victim of a scam, but if you were - report it to law enforcement, and don't make the same mistake twice.
Best of luck to you.