It ceased to be a hotel in 1935, as it was short of facilities by 1935 standards (e.g. no private toilets, and only nine bathrooms for 400 bedrooms), and would have been very expensive to upgrade due to being essentially carved out of single lump of stone. It was renamed St Pancras Chambers and used as railway company offices, most recently by British Rail, until it was closed down in the eighties because its fire safety certificate was revoked. Since then it's been empty, although used as a film location on numerous occasions. The outside and a small part of the interior had a complete (and very expensive) restoration in the late nineties, and it looks good from the outside.
As part of the redevelopment of St Pancras for the Channel Tunnel link, Marriott hotels are going to use part of it as a new railway hotel, with a more modern extension built on the back. Some of the rest will be used for loft apartments. But for the next year or two, you can still go and see it as is. I didn't have my camera, so these are low quality photos from the phone.
It's currently owned by LCR, who run the Channel Tunnel link, and have information here.
The facade, seen from inside
The only survival of a number of painted murals
Intricate stone carving
The ceiling of the grand staircase (which was restored in the 90s)