Surface Pro 4 type cover suddenly not working.

Ok I have it working again.  This time it survives a reboot, closing the lid, shutdown, etc.

I also now get the proper "Do want to switch to tablet mode" prompts, etc.

And, the rotation now works correctly in Tablet mode (it did not work at all prior).

Now being a sys admin, I normally only change one thing at a time, but I didn't follow my own rule and I made two changes.  Not sure which resolved it, or if both did.  But anyway here is what I did:

The keyboard is disconnected and the Surface is in Tablet mode. I have a USB keyboard and mouse connected.

1. Downgrade UEFI firmware to v103.2851.256.0.

http://www.catalog.update.microsoft.com/Search.aspx?q=Surface%20-%20Firmware

and look for that version.

To install it, go into Device Manager, look in the Firmware section, and update Surface UEFI with the file(s) inside the .cab file.

You may need to modify your registry to allow downgrades.

Reboot.

2. Uninstall the "I2C HID Device" in Device Manager, under Human Interface Devices.

Scan for changes.

Reboot.

Power down, Power on.  (I did a full power cycle as an afterthought, since I know that some firmware updates require full power down, and many changes require two reboots)

Picked up the tablet, turned it sideways, and it switched to Portrait mode.  Yay! Then I connected the keyboard and it immediately worked. Doubleyay!  I went through various combinations of shut down, power cycle, hot/cold disconnecting keyboard, etc.  and it still works.

@ElegantPen, I do not have Hibernation enabled, that was one of the first things I disabled when originally purchased. That might be a different issue, although glad it worked in your case.

still happens sometimes after a windows or firmware/drivers update - but here is how to reliably fix it every time (only if touchpad still works in UEFI/BIOS):

• only once: update windows
• only once: install latest surface firmware/drivers (https://www.microsoft.com/en-us/download/details.aspx?id=49498)
• every time: check if Sticky/Toggle/Filter Keys are not enabled (disable them)
• every time: perform two button shutdown, and then two button startup (hold 30 seconds)
• every time (most common problem): check if your "usb input device" wasn't replaced by "wacom device" driver, then just simply rollback or manually update (https://answers.microsoft.com/en-us/surface/forum/all/solved-surface-pro-3-type-cover-working-in/9046ba77-ef3f-4be9-82bb-ce44182b3c26)
• every time (some blame hibernate for this problem): detach type cover and reattach only after full 24 hours, make sure to also fully drain the battery on tablet before that (nothing shows on display when you press power button), then charge again

I can confirm that this does NOT work and is NOT a solution.  It is simply the same thing copied and pasted from every other thread, including this one.

still happens sometimes after a windows or firmware/drivers update - but here is how to reliably fix it every time (only if touchpad still works in UEFI/BIOS):

• only once: update windows
• only once: install latest surface firmware/drivers (https://www.microsoft.com/en-us/download/details.aspx?id=49498)
• every time: check if Sticky/Toggle/Filter Keys are not enabled (disable them)
• every time: perform two button shutdown, and then two button startup (hold 30 seconds)
• every time (most common problem): check if your "usb input device" wasn't replaced by "wacom device" driver, then just simply rollback or manually update (https://answers.microsoft.com/en-us/surface/forum/all/solved-surface-pro-3-type-cover-working-in/9046ba77-ef3f-4be9-82bb-ce44182b3c26)
• every time (some blame hibernate for this problem): detach type cover and reattach only after full 24 hours, make sure to also fully drain the battery on tablet before that (nothing shows on display when you press power button), then charge again

Had this issue a few weeks ago. My SP4 Type Cover suddenly stopped working and screen rotation didn't work either. Tried all kinds tricks to get them working, but nothing seemed to have an effect. 

Then I remembered that I had recently enabled Hibernation mode from the power options, which I have not been using at all before. I disabled it again and let the surface sleep on my desk plugged in for an hour or so. When I came back to it the Type Cover and screen rotate were working again.

I'm not sure if the Hibernation mode was the cause behind this. However it seemed to have fixed the issue for me. Haven't had any problems connecting the Type Cover or rotating the screen since then.

Experiencing the same issue. It seems many users have been duped out of money through no fault of their own. There was a similar issue regarding flickering screens in SP4 but eventually MS started a replacement program for that. If anyone knows how MS got convinced to replace them, perhaps we could do the same.