hi, can you see any lights flashing for lots of drive bays, or are they all a solid light (ideally green) and just that particular drive bay which failed, is flashing/blinking red?
also can i check how many blue lights are lit up at the bottom, and what does dashboard show you for used and free space, in values and % when you get a moment?
if nothing else has changed (such as a software upgrade, or firmware update etc), then it could possibly be a faulty cable, or some drive issues, of which 1 seems to have failed, with flashing red light?
if a drive has failed, and if the other drives are flashing yellow/green and rebuilding, then the rebuild process usually takes about 1 day per 1TB of data that you have, but in some rarer cases it has also been known to cause the drobo to be inaccessible to the computer and / or to dashboard sometimes.
it is probably worth creating a ticket with the support team (especially if you are still in support), as they may be able to help confirm in case any other drives are having problems, or in case there is a hardware issue or somesort, (but i think you can still raise a pay per incident, even if out of support, if you would wish)
if nothing is accessible at the moment, then you could try fully shutting down the computer, and then restarting the computer (just in case that makes a difference)
if that does not work, and if no other lights are flashing, and are solid green, then its possible that removing the failed drive (while drobo is on), could alleviate things, and then adding in a blank/empty/uneeded working drive to replace it, (also while on) could initiate a rebuild to use the replacement drive, and then could get things back to a working/accessible state, after that process has comlpeted.
alternatively, you could try shutting all down, and then booting up the drobo (but without any computer connection cables), and then seeing if it goes through the usual boot up sequence with blue lights from 1 through to 10, and then to see if it goes into standby mode.
(if it seems to hang during the blue lights, then please do try to allow lots of time here as it may be self checking or correcting things during this process, but please try to check in case it starts rebooting and looping during its boot up sequence)
once it completes booting up and goes into standby mode (ideally), then you could try connecting it to a freshly-booted computer, with dashboard again… how are things now?
in case the same issue is there, (or especially in the case of a reboot loop) you could try putting the drobo into a readonly mode, for example as mentioned here, to see if things stabilise for you to at least access the data to copy somewhere again, though if you do this before any drive swaps, then please remember to take it out of readonly since it can not actually rebuild when in readonly mode)