Liqui.io withdrawal history shows my transaction as completed but if I try to check Txid through etherscan.io, it says "Unable to locate Transaction entry". Also Bittrex wallet went
down a couple hours ago and it now shows "Mysterium Wallet Offline : Blockchain congestion detected." Does it mean my sent MYST are lost? I typed correct wallet adress btw
No. It just means that your transaction was never confirmed on the blockchain before the wallet went down. Give them a few days to sync their MYST wallet again, and all should be well once their wallet catches up to the block where you sent your transaction. Sometimes, a good sync could take a few minutes, or it could take days, depending on the entire size of the block that's being synced.
Your coins should be fine; just give it a some time, and make sure to check the status of the Mysterium wallet. If it stays offline for a bit, make sure to look out for news from the Bittrex team, and if there is none, open up a support ticket.
Hope this info helps.