I sent monero from Poloniex to my GUI wallet last week via two transactions. Both show as complete transactions on Poloniex. Neither one has shown up in my GUI wallet.
What can I do?
I believe this has been a Poloniex issue for quite some time. If you did not use a Payment ID, you may be screwed.
Try to find your payment on the blockchain: https://monerohash.com/explorer/
There are other sites for exploring the chain https://getmonero.org/community/merchants/ (Look under Block Explorers). I do not know which is nicest or best.
You may want to get in contact with Poloniex. Another option may be to load your wallet from seed on the wallet-cli or perhaps mymonero, to verify that it is in fact in or not in your wallet.