The cards data probably became corrupted. I had removed mine for a similar issue. When I went to load YouTube recently, it complained I should use an SD card so it would not need to reload the channel.
I tried to put it back in, but it would freeze on 'Mounting SD Card'. I put the card in a PC, which couldn't read it (it's probably Linux efs format, not something Windows can read), but asked to format it. I let it do that, then put it back in the Roku 4. This time, it asked if I wanted to format it. I let it format (takes a long time for 32G), and it seemed to work. I'll see if it holds up over the next few days.