It happens. As other commentators marked, Poloniex is a reputable exchange with pretty long history and solid reputation. I seriously doubt that they would try to fool customers only to get a few extra hundreds or thousands dollars. The problem may be with your account – big exchanges try to meet KYC/AML requirements because they don’t want to face problems with governments. Maybe, your activity was suspicious or something like this? Anyway, it’s better to contact Poloniex customer support once again (on Twitter or other social networks) and then simply wait a bit. I’m sure everything will be OK.