I got this great news from BitTrex yesterday:
"Maintenance is complete for NEM/XEM. Thank you for your patience."
I have not logged on yet to verify, but expect to later today.
I've logged in my account and can confirm Bittrex XEM wallet is online for both deposits & withdrawals. Good news there and now XEM trading volume on Bittrex has spiked considerably, #2 behind #1 Upbit.